Your one and only clue: Find him in his natural, underwater habitat!

So that would make us all belive that you can find him on the H2O map.
You want to start a server with the H2O map, and then make sure that you set a password so not everyone can join.
Because it will make this much easier as a solo monster, so you can go and explore.
Since this place is a little far from where you come during normal gameplay.

Once you are in the game you need to go and stand on launchpad nr. 3.
And then face the same way as you can see me do on the map.

When you have found launchpad nr. 3 and got yourself in to the right position, you want to aim for the big rock out in the distance as shown by the red circle.

Once you walk all the way out to the big rock, go around to the back and you will find this little ball.
And inside that is the Dopefish

When you walk up to the ball you will see this little purple message above it that reads: The Dopefish Lives.

When you Get that message you will also get the achievement.
Hope this helped you guys out.