Who was the Greek god of the sea?

Poseidon is the God of the Sea (The Mediterranean) and the Friendly Sea (The Euxine, or Black Sea) as well as underground rivers.

Ocean, a Titan, was Lord of the river Ocean, a great river encircling the Earth. (Thought to be the Atlantic)

The sons of Ocean and Tethys were the Gods of all the rivers on Earth.

Nereus was a Sea God.