You may have gotten one of these out of a serial box as a kid. You put it in a sealed pop bottle full of water. When you squeeze the bottle it will go down, when you release it, it will go up. I may come up with a fancier version at some point, that looks more like a sub.
I printed it with a raft, but no supports, this is important, if you use supports it will print thme in the inside which will make the object too dense.