What are the gods of water called?

Well, this is a difficult question to answer completely, but I would venture that they might be called "Hydradea". "Hydra" translating in Latin as water, and "Dea" translating to god.

As for actual gods names, the most prominent and well known god of water is "Poseidon"

or Chthulu if your into the idealism's of H.P. Lovecraft.