All 5 methods are pretty useful and produce a lot of monies, they are the fastest of all, that i tried to find. They're very useful on the speedruns, forth and fifth metods are the most useful of them

First two metods locate in the Tan Line Temple
Method №1
There is a spot in the middle of the temple, below the purple breakable blocks
If you head to the right, you can find a secret room with 8 big pots, that give you 80 monies.Than you can simply reset them by re-entering this room with a door on the left. It takes about 25-30 sec to get 1 80-gem run.
Only minus is long from-ship-to-ship distance. It takes about a minute to get to the place, and a minute back.

Method №2
Another spot in Tan Line Temple located above the long chain room, where the first key are.
If you jump, right there the chest, you'll find a chain, that's lead to a secret room. There are 6 big pots.
It takes less time, than the previos method, about 10-15 sec, but has the same problem.

Method №3
Firstly you need a Risky's skimitar to perform the trick. The spot location is the second area on the Spidreweb Island. There you can find 2 infinitely spawned zombies.
Every time you hit them in the head, they give you 1 gem. If you hit it with hair, head goes far away. But if you bounce on them with scimitar, you can hit it about 5 times, 2 zombies mean double gems. You can grind about 100 gems in a minute.
But it the slowest method of the four

Method №4
The fourth method is my favorite. It's quick, easy, and close to the ship.
The spot locates below the mummy statue on Saliva Island, area, where the heartsquid is.
At least you need a Risky's hat
There is a wall on the right with a secret, which contains 3 big pots. You can easily break them through the wall, and colect gems with a lamp. Then you can reset it with re-entering the area. With Risky's canon, method can be performed in 5-6 seconds.
After collecting 999 monies, you can simply use the Pirate flare to go back to ship

Method №5(new!)
The fifth metod i found not a long time ago, i saw it using Tky619 on his speedrun, so thank you about that. So the spot is a ghost room in the Village of the Lost Souls, not a long before the red coin location.
On the very bottom of this room there are place, there are 3 big pots, that is wide open, but invisible. You can easiely shoot through that place to destroy these pots, and collect monies with lamp.
The fastest way, takes about 2-3 second for a 30-gem run, but you can't use Pirate Flare immediatly