Ok for this you will need a base! This is becuse of a glitch in the game as of now, your underwater bases still have water in them which can be seen by building one part at the top of the water. THIS IS NEEDED TO HAVE A PET!

You need to catch your pets :D do it with something that cant kill you (peepers, airsacks are at the top of my mind) So once you got yours you got your pets now for step 3!

For this last step relese them into a part that is underwater of your base they now can swim around See? If you cant there is a peeper in the cornor he is alive! From the glitch I talked about.

Ok so there adding a aquarium! If you can put fish in it YOUR DONE :D

If you live on the island (dont ask if you dont know) you can flood stuff there create a tube
add a hatch go inside place a peeper or whater fish you wnat and put some windows on it :D you can even break take the hatch off and put windows on :D aquarum!