The Greeks weren't the only ones with a god of the sea. Poseidon is in good company. He sits at the center of the lunch yard and mingles with all of the jocks and varsity team captains. No one would want to mess with Poseidon and his huge temper.
Mannannan Mac Lir is the Irish Celtic god of the oceans. Like Poseidon, he was in charge of making the weather and of calming or disturbing the seas. Unlike Poseidon, he was most always cheery and happy. He liked to make people's wishes come true. Read more about him here.
Tangaroa is the Maori ocean deity. His mother, Papa, was goddess of the earth. Her body was so full of water that she burst, creating the oceans. Tangaroa appears as a fish and created all of the fish and sea creatures that roam the oceans. Some sailors today keep a piece of brain coral on their ships in the hopes of appealing to Tangaroa for a safe journey across the seas.
In Norse mythology, there are lots of deities that have to do with water and the oceans. Njord is one of the biggest of these cheeses. He is a Vanir god and father to Freyja and Frey. He governs trade and safe passage on the seas.