[ooc: aww *hugs* I know, that is annoying. Also sorry, Michiru is so going to break his brain with this at some point lol]
I suppose the many of them don't separate water types, but also consider the fact that in Ancient Greece, Poseidon specifically was the God of the Sea, and they believed in nymphs for different regions. For example, naiads were nymphs who watched over streams, nereids were nymphs of the sea, and so forth. And the Romans, who gave Poseidon the name Neptune, took from the Greeks.
You can stick with the obvious four, but it may or may not answer all the questions so you may want to take a guess with aether.