The helm is propably the most complex room currently in the game. From there you execute diving maneuvers which can make or break every strategy. You will spawn in this room. The hatch above you will get you to the front deck, the one below you to the torpedo room and the one opposite to the map to the Captain's/periscope room.


The map
I'm going to skip all obvious parts such as the distance indicators and the degree-based orientation system (which orientates itself to the North, not the direction of the ship). This map will highlight any objects of interests (mines, incoming torpedoes, ships, mortar-fired explosives, incoming active SONAR pings). Please keep in mind that the map relies on the hydrophone to spot ships if the navigation assist is off, so if the hydrophone hasn't checked on a target for quite some time, its location on the map might be inaccurate or disappear. Also keep in mind that planes will not be shown, so when surfaced, you should always have a guy on the outside to check on things.


This display, located a bit to the left between the map and the rudder, will display the sub's, the periscope's and the hydrophone's current orientation. Only really useful when nobody tells you where a sound is coming from.

When starting a game, the rudder will start out in automatic mode, which is when the big yellow screen on it is on. In that mode you only need to point the tiny needle on the rudder to the direction indicated on the yellow screen to go there, and the sub will do the rest. When flipping the switch on it, it will change to manual mode, where it acts like a normal steering wheel/rudder, The display on top of the rudder shows the current position of the rudder. Please keep in mind that in manual mode the rudder position won't return to 0 automatically, so when in manual mode, do not leave the helm or switch to automatic mode, otherwise you might leave your sub spinning in circles.


Depth Indicator
Pretty self-explanatory, apart from the color coding. The blue zone is periscope depth, the green one is the zone where you hull takes no damage, the yellow zone is where your hull slowly starts to take damage, and the red zone is where the hull will take damage rapidly.

Depth Under Keel Display
EDIT (4.9.2020): Apparently 20m under the keel now brings up good results for hiding. I will update this guide once I can confirm that myself.

A bit to the right of the Depth Indicator is the Depth under Keel Display. Pretty much self-explanatory. If you decide to hide near the bottom of the sea, make sure to get as low as possible, as even 10m apart from the ground will result mixed results when it comes to detection. You can also touch the ground safely if you don't dive too fast.

Diving Console
This console is crucial when it comes to fighting. Diving can save you by widening the distance at where the destroyer cannot detect you, which gives you some time to avoid its depth charges. The right lever will fill the ballast tanks, which will make your ship sink and the left one will drain it with air, making the sub rise. Keep in mind that to drain the tanks you will need peressurized air (display on the upper left shows how much you have left). You can only create more on periscope depth. The wheel will change the position of the sub, which can make your ship rise or dive temporarily or assist the sub's natural direction, with the aid of the sub's speed. Keep in mind that this will affect the buoyancy display (upper middle), as the sub will rise or fall into its natural depth after you default the sub's position.
The ballast tanks' current fillage will indicate what the sub's natural depth is. The big block indicates periscope depth, the first big mark 25m, and ever consecutive big mark will add 40m, so the next big mark will be 65m, the one after that 105m, etc...

Sea Floor Map
this map will display the form of the ground infront and behind you, relative to the sub. Very useful if you are lucky enough to spot a location where you can hide in. Keep in mind that it may change if you change direction.

Speed Regulator and Indicator
Self-explanatory. The speeds will vary based on where you currently are. On the surface you will reach a top speeds of 18knots, under water a top speed of 6 knots. Full speed will reach top speed and every following tempo will be half the speed of the last one. Keep in mind that the current hydroplane will affect the speed.

Fuel, battery and oxygen supply
Located next to the speed indicator. Shows your current supply of things. Currently the only thing worth watching is the battery charge (middle dispplay), as you have so much fuel that you can't waste it all and you will refill oxygen every time you get to the surface or use the snorkel.

The welder
Located on the door to the Captain's/periscope room. It's used to weld leaks around the sub. There is only one welder, and it doesn't respawn, so if someone that does has it in their pockets, it's gone, so make sure to return it after use.

This room is located under the helm. The only way out is the hatch on the ceiling.

There isn't much to explain here when it comes to reloading. The red lever on the side of the tube will lock the tube when you pull it down and unlock it if you pull it up. The big lever on top of the tube will flood the tube when the lever points to the center of the room, and will cease to do so when pointing towards the nearest wall. You will know when it's full when you don't have any flowing water and/or when the display on the tube's door indicates that the tube is full. Keep in mind that it will flood the room instead when the door's open. The door itself can be opened by pulling the handle on it, if the door is unlocked of course. Torpedoes are being loaded by pulling them into the tube. Once a torpedo is fired, you need to drain the tube by pulling the draining switch on the center console for the fitting tube. Make sure that the flooding lever is pointing towards the wall before draining and don't open it until the tube is completely empty (display showing that it's empty).

The captain's room (also referred as the periscope room) is to locate targets, lock onto it and fire at it. The metal door leads to the helm, the wooden one to the engine room. The hatch on the ceiling will lead to the upper deck.

The Hydrophone
The hydrophone is located on the right side of the metal door. By turning the wheel you change the direction of where it's pointing at, seen on its yellow display. Be aware that its direction is relative to north, not relaitve to where the ship is pointing at, so it will always will listen in one fixed direction, even when the sub is turning. When pointing towards a target the little display on top will point more to the right the more accurate the hydrophone is pointing at it. When pointing towards a target the hydrophone will create a noise, which is different for every type of ship, although it does takes practice to tell which one belongs to what.

The periscope
The periscope is located in the middle of the room and can be accessed by pulling down. Note that this will not extend the periscope itself.
Once you have the periscope ready, you'll notice two levers. The one on the left will extend the periscope when turned upwards, and will retract when turned downwards. Keep in mind that you can look through the periscope without extending it when the sub is on the surface, so don't extend it, as it will increase the detection radius. When underwater, the periscope will only work extended in periscope depth. The switch on the right adjusts the zoom. When pulled downwards, it will increase the zoom and pulling it upwards will decrease it.
In the middle of the periscope there are two switches. The upper one will lock a target you are aiming at. It will only lock the target if the green lamp next to it is lit up. The lock will last about 3 minutes before it needs to be redone. The lower switch locks the viewport in place in order to help players who cannot move around much. You can still spin it around, but you don't have to move around the periscope anymore when the viewport is locked.
Also, when the viewport is not locked, you can see to the front of the ship by turning it so that you basically look at the metal door, if the periscope wasn't in the way.

Targeting computer and torpedo launching equipment
On the left side of the metal door is the targeting computer. It will display the locked on target, if one has been locked. The levers underneath it will adjust the crosshair, which is where the torpedoes are trying to travel to. These come in handy when the target is moving in an unexpected manner, as the crosshair will get more and more inaccurate if it's not adjusted. The displays around it are not very useful at the moment, except for the targeting solution. It will display how accurate the solution is, at least for the moment the target got locked. The accuracy of the solution depends on how much the sub deviates from the target, as the torpedoes have a rather big turning curve. Left of the targeting computers is the torpedo launching equipment. It tells you what tubes are ready and lets you fire them at your locked on target by either turning or pulling the switch down.

• As I already said, torpedoes have a rather big turning curve, so the periscope's best position is pointing straight forward and 45° to each side. The other directions make the targeting solution too inaccurate to be viable. Therefor the driver needs to aim, rather than the one on the periscope.
  
• You can stow away the periscope (don't forget to retract it!) after you locked your target, as 3 minutes can be a pretty long time, and sometimes diving is a good solution to evade counterattacks. Keep in mind that even under periscope depth torpedoes will work, as they always surface. It will just take a longer time for them to get to the surface the deeper your sub are.
  
• While it's tempting to fire all torpedo at once to give out one giant blow, it's better to fire them individually with a slight delay between them. That way the damage will come over time, but you have a better chance to score more hits.
  
• When you are under attack, it's good to have an extra person in the periscope room to handle the torpedo firing equipment, as the targeting computer's crosshair can move pretty fast and it can be hard to catch up if it's too far away.

The engine room, even though it holds great potential in micromanagment, it's one of the less thrilling rooms, as there's not much to do, and what can be done is either very simplistic or rather tedious for some, and also can be the fatal cause for a mission to fail. The only door leads to the Captain's/periscope room.

The Snorkel & Power Switch
The snorkel is a tool to let the diesel engine work and get air underwater. It can operate in periscope depth, however will fail once you get deeper. By turning the wheel you can deploy and retract the snorkel. Whether it's deployed or retracted and/or operational is highlighted by the three displays above the wheel. Currently there's no downside of using a snorkel, so I recommend leaving it deployed for the rest of the game.
The power switch will alternate the power between the battery and the diesel engine. The diesel engine can only run on the surface, or with the help of the snorkel, in periscope depth. The switch will automatically switch to battery once the diesel cannot operate anymore. While it's tempting to use battery power when sneaking up on enemies, it needs to be said that the diesel engine won't make your sub more noticable, and the battery drains rather quickly, and considering how you need the battery for below periscope depth travelling, it's best to stick to diesel engine as long as possible, as the engine fuel won't even run halfway out even after about an hour of play. Keep in mind that if the engine assist is off, the switch has to be pulled to diesel engine manually, otherwise you might waste battery power. The battery will be recharged by excess power.

The energy regulator and pressurizer
The left lever will regulate how much power is being used by the system, which is mostly just things like light. Be aware that the light will only dim for the host, and not the clients. The display above it will show how much power is currently needed (red needle) and how much power is generated (black needle). The black needle will only show the power output of the diesel engines though.
The pressurizer creates pressurized air, however it requires the snorkel to be deployed and the sub to be in periscope depth to work.

The left lever will influence how fast the diesel engines are running and thus how much power is being generated while the right one will determine how fast the engine is being cooled. Currently the punishment of overheating/undercooling your engines is just a decreased power output. On the right side of the lever you will see the current speed, but that's not often important. A small trick on when you notice that you can't really handle the engines anymore is to put the engines on full power and the cooling to half. That way the engine will barely undercool or overheat.

The engine block
The small little gat will lead you to the engine block, which doesn't serve a purpose at the moment.

There are two decks: the front and the upper. They both have hatches on the ground. The one on the upper deck will lead you to the periscope room and the one on the front deck to the helm.

The front deck features a .50 cal machine gun for anti aircraft use, aswell as a 80mm cannon, which must be loaded manually and can only be moved with the wheels on top and the side of it. Spinning the wheel on top of it will turn the whole deck, including the machinegun. The display on the 80mm cannon displays targets that are in range of the 80mm cannon (within 4000m). The shots fired from the deck gun or the machine gun doesn't have falloff, so you won't have to account for the trajectory. One 80mm shell will cause as much damage as 1/5 of a torpedo. The anti-aircraft machine gun is hindered by the 80mm cannon, which means that you often have to turn the deck around in order to hit incoming planes that come from the direction your cannon is currently pointing at. Also be aware that the machine gun can damage the hull, aswell as the 80mm. Overall the front deck is good for the driver to take quick peek outside to assess the situation or for someone to assist on shooting down planes. However it's a pretty bad spot to look out for planes themselves or to spot ships.

The upper deck only features a single machine gun for anti-aircraft purposes, but offers more freedom when it comes to observation. There is a gate leading to a unarmed platform which is great for spotting enemies and planes, as the machine gun can get in the way of the spyglass when you observe on the main platform. Please note that the spyglass actually will show you things further away and isn't just there to zoom in on things you could have spotted with your bare eye.

• The decks are really useful to see the enemy's formation and the ships used. You have the same distance as the periscope when you use the spyglass, yet have more freedom in terms of observation.
  
• Make sure you listen carefully. Bombers are hard to spot with the naked eye sometimes, but its deep humming can be heard from far away.
  
• when your driver decides to dive and you can't get back in in time, you will be automatically teleported inside, so don't panic when you notice the sub sinking.

Detection plays an important role when it comes to taking down enemies, especially heavier ones such as the battleship.

Detection itself is visual-based, meaning making more noise won't make you more detectable, only making you more visible. How far you can get detected is visible in the helm, as the map will display a solid white circle that depicts the detectable radius, with the limit being 4000m. With that being said, here are the things that determine your level of detection:

Depth
EDIT (4.9.2020): It seems that the current detection range at night has gotten much lower, with players even being able to do a close surface without getting detected. I didn't test it myself yet, but will update my guide accordingly once I get more detailed numbers.

The depth will determine your base detection radius. The deeper you go, the less visible you become. I'm not sure how deep you will have to go, but about 25m or lower, the radius will drop to 0m (undetectable). On the surface you will always have a detection range of 3000m and the only thing that can influence it is the periscope and firing weapons.
At night your radius will be around 500-750m.

Speed
Author's note: I haven't done much reasearch independently about this topic, so some information in this segment might be inaccurate, as I'm trying to write from memory without any notes. I will update this segment once I find time to experiment more.

EDIT (4.9.2020):It has been brought to my attention that the following rest section about speed is a bit obsolete. Apparently it's rare to get a detection range over 1000m when at periscope depth at all and while the periscope is down. So take the following section with a bit of salt, until I have done more in-depth testing about it again.

As already stated, on the surface speed won't affect your detection range. Once you get in periscope depth however your initial detection range is about 500m. Full speed will get it up to 2000m, and the other speeds are inbetween. Half speed should push it back to 1500m and slow to 1000m. Speed won't affect the radius anymore however if you dive deep enough to get it to 0m(undetectable).
I did not collect any info on how speed affects the detection range at night yet.

Weapons and the periscope
The periscope itself will add at periscope depth 2000m, but the detection range will, as already stated, cap out at 4000m. The influence of the periscope however will get weaker very fast once you get below periscope depth. The snorkel, which also works on periscope depth however will not raise your detection range in any way. Weapons, such as Torpedoes, the 80mm cannon and the AA machineguns will always bring the range up to 4000m every time they fire.
At night, the periscope will only add about 500m to the detection range. Weapons will still make the radius spike to 4000m every time the fire though.

After you get spotted, the element of detection does change. While enemies can still make out your location when they are in the detection range, but will also use active SONAR to track you down in case you try to dive deeper. This active SONAR will reveal your location every time you get pinged. However the active SONAR has multiple blind spots.
One very commonly used one is depth at close range. Ships cannot track you when you are too deep at a very close distance, as the SONAR will just pass over you. The deeper you go the bigger the radius will be. If you go down just in time, the enemy will even lose your location and cancel its attack, but keep in mind that diving deeper can cause hull damage and/or make you more defenseless, as torpedo will take more time to surface, and thus you won't be able to hit at close distance. Furthermore you will have to get back to periscope depth to relock a target.However diving deeper thus also allows you to change course and get away from the enemy, avoiding potential depth charges.
Another blind spot, which is rarely used due to the game's nature with the sea floor, is the ground. Once near or on the sea floor, the active SONAR is unable to detect you, letting you avoid further tracking. Please be aware though that most of the sea floor is in the red area and thus getting to there will cause major hull damage and won't let you sty there for long. Furthermore, when on the ground, your ship will be unable to move, and if the enemy is deploying depth charges, you will be a sitting duck.

EDIT (4.9.2020): Apparently hiding at the sea floor is a more viable tactic now, as sea floor shallower than 100m is more common now, and hiding is easier, as 20m between your keel and the surface is enough to be hidden from the sonar. Please be aware that this won't protect you if you are still getting hit by depth charges though. Furthermore, damage at 120m doesn't seem too severe now, so diving deeper to escape the charges is also more viable, aswell as reports of people rarely getting hit by depth charges at >100m depth, if at all. I will update this section accordingly once I can check this for myself and also form new strategies.

Freighters
Freighters are leightweight and often unarmed, and if they are armed, they only have a cannon which they will fire once they detect you, if they have cannons. The cannon, as all enemies do, will miss the first few shots before hitting, which gives you some time to dive. They do not make any attempt to escape, so they are generally easy targets. Take them down with either the 80mm cannon or the torpedoes. They take 1 torpedo or 5 80mm shells until they sink. This ship type should be always the least important priority target when attacking.

Destroyers
The destroyer is a grey ship with 4 stovepipes. It is equipped with cannons and depth charges. Once detecting you, they will turn towards you, and either circle you to fire its cannons at you if you decide to stay on the surface or drive over you to use their active SONAR to track your location and drop depth charges once they are over you. Even if they lose your position they will still drop depth charges if the last known location was detected recently enough. Their SONAR however only works in the direction they are going, so after driving over you they continue to sail about 1000m and then turn to locate you again. They take 5 torpedoes or 25 80mm shells (1 torpedo = 5 shells).
The best strategy is to lock the target and fire all 4 torpedoes at it while diving to 100m simultaneously to get the best chance of avoiding its SONAR. Keep in mind that once you fire the torpedoes, it will inevitably turn, so keep that crosshair on it, and since you won't be able to lock onto it after that, let someone handle it without them doing anything else. Head the destroyer head on, and if you are able to load the remaining of the 5 needed torpedoes before it's 1500m away from you or closer, fire at them as fast as possible. If not, hold fire and wait until the pinging stops, then change the course 45° off the course of the destroyer in order to avoid potential depth charges and/or reduce its damage.
If the destroyer is not destroyed yet and the depth charges did explode (or did not drop at all), dive up to periscope depth, turn the ship around to face the destroyer, lock the target, and fire the rest of the torpedo. Keep watch if there are other destroyers though.
Destroyers will sometimes guard freighters in a convoy and will either lead them or follow them. They can also be found in killing squads, which consists of 2 destroyers and 1 battleship. At night they will fire flares in order to track you, and once the 2nd flare hits the water they will begin to fire or to track you down with their SONAR.

Battleships
Battleships are the toughest and hardest enemies in the game. They take 9 Torpedoes or 45 80mm shells (1 torpedo = 5 shells). They look like destroyers, but are flatter and darker. They are equipped with cannons, depth charges, turnable active SONAR, front fired torpedoes (which can only hit when the sub is in periscope depth or higher and have a similar turning radius as the sub's) and a hedgehog mortar, which can be fired at a distance of 1500m, although they tend to get more distance before they use it.
When detecting the sub, they will either circle the sub and fire their cannons if you decide to stay on the surface, or will begin to escape when you dive in order to get in range for the mortar. It will rarely use depth charges, and only if the opportunity is right. The mortar will fire many explosive shells over a large area that are equipped with contact fuzes that will detonate on impact and will work at any depth. A shot of a hedgehog mortar is pretty much unavoidable, and often results in a mission failure if the battleship manages to engage its mortar. Because of this, the battleship is your most important target.
If it is alone, sneak up on it as close as possible, quickly dive up, lock the target and get below periscope depth to avoid its torpedoes. keep the crosshair on it while firing as many torpedoes as possible at it. Continue to go full speed, and maybe try to get up to periscope depth once it's completely turned away from you. (Author's note: although untested, it could work to get to the surface and stick up there for a while to make the battleship turn and thus gain some precious time. This is untested though)
If it's in a killing squad, do not sneak up on it. Rather attack it as far as possible, and before you attack the destroyers. The battleship will stay put, yet ping you, whereas the destroyers will try to hit you with depth charges. However as long as the destroyers are on their run to you, the battleship will not fire its motar, giving you precious time to bombard it with torpedoes. After locking it, dive to 100m in order to avoid depth charges as best as possible while firing torpedoes at the battleship. Once destroyed, take care of the destroyers.

Fighters
Fighter planes can be heard with a light buzzing sound when on the deck. They will appear whit-ish at first, but later turn darker once it's coming down to attack the sub. Fighter planes will only spot you when you are on the surface, and will ignore your detection range. They are relatively easy to deal with, either dive or use the machine guns and aim at where they are going to be (not where they currently are!), and only once they become darker. Be aware that one plane often means that are more to come, and often a torpedo bomber will appear later on too.

Torpedo Bombers
Torpedo bombers are sometimes hard to spot, but they have a deep buzzing sound when they approach the sub. They take a lot more hits until they crash, so it's best to dive and let the gunners try to shoot it down before it drops its torpedo. If it's shot down before it drops its torpedo, it's safe to get up, but be aware that another bomber might come soon after. If the bomber wasn't destroyed however or dropped its torpedo, dive below periscope depth to avoid its torpedo.

Forts
Forts are very distinguishable from ships, as they are basically platforms on the sea. They are not armed nor do they move, and take 1 torpedo or 5 80mm shells to destoy. They always will come alone or with planes, so only make sure to watch out for planes.

Special Thanks to:

Das Bot

• for helping to spot outdated and/or inaccurate info.