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The Ichthyocentaurs, a pair of centaurine sea-gods with the upper bodies of men, the lower fore-parts of horses, ending in the serpentine tails of fish
Leucothea, a sea goddess who aided sailors in distress
Nerites, watery consort of Aphrodite and/or beloved of Poseidon
Nereus, the old man of the sea, and the god of the sea's rich bounty of fish
I really liked the original idea of Zeus. That's about as powerful as you can get, over the oceans and everything else. Or you could put all the other fish in pairs, and call him Noah.