Fish

A fish is a general term for various species of scaled aquatic animals.

Captain Jean-Luc Picard kept a lionfish, a type of fish, in an aquarium in his ready room aboard the USS Enterprise (NCC-1701-D).

Idiomatically, feeling like a "fish out of water"... Montgomery Scott, after his rescue from the USS Jenolin seventy-five years out of touch with technology felt like a fish out of water.