Poseidon

In Greek Mythology, Poseidon was the name of the God of the Seas. He was the son of Chronos and Rhea, brother of Hades and Zeus. Only after Zeus had established his reign as supreme deity, did Poseidon become the God of the Seas. He was part of the original twelve Olympian Gods. Unlike his brothers, Poseidon had a "good" contact with mankind.
(Note that there are parallels with EN.KI who as EA became the Lord of Water after EN.LIL was made the leader of the Anunnaki. - There also is a probable link to Sirius).

In Roman Mythology, he was known as Neptune.