So, Poseidon isn't the god of all the Oceans?
I would have to assume not, if only because thus far the Greek's arent that Ascendant as a whole. I figure it works a bit like how the Fae courts are described in the southern hemispheres, with "relationships there consist more of strong alliances and consensus influence among a much larger population".
Or, maybe the Ocean is divided up into more geographic territories, a formalized Seven Seas or something with actual Borders?
Or, maybe none of the Human deities ever really gained supremacy under there, and there's a race of hominid offshoot fish-people (call them "Bigfoot of the ocean") chilling in Atlantis with Aquaman. Or a deep-sea Dragon that never gave up its Domain, like a Pyrovrax down near the Volcanic Vents?