I actually suspect that the majority of life - and intelligent life - in the universe is probably ocean-based. If ever space aliens visit us, unlike the movies, I suspect their spaceships may be more likely to be like sealed aquariums than an air-breathing setup. I've never seen this idea reflected in science fiction though.
In the later books of the Lost Fleet series, they come across an alien race that is aquatic, with ships that are much more maneuverable than human ships. They theorized that the ships were in fact filled with water which allowed them to make more radical movements.