What kind of under water animals could be pets?

Some kinds of fish, such as guppies, goldfish, and platties make good pets. I would not suggest glofish because they are very suceptible to a disease called dropsy, caused by poor water quality. Dropsy is almost always fatal. African dwarf frogs make good pets. Although amphibians, these small frogs stay in an aquarium and occasionaly swim to the top of the tank for air. If you get african dwarf frogs, you should get a tank lid to keep them in. Look around your pet store for other options.