Who is the Greek god of the sea?

Poseidon, who drew the sea when he and his brothers divided the world after overthrowing their father - Zeus took the sky, Hades the underworld, and the earth was held in common. He is also god of earthquakes, called the earth-shaker, and of horses, which is a stranger combination until you notice that both are things that move under you without warning.

He is not a benign figure. In the Odyssey he is the antagonist, keeping Odysseus from home for ten years because he blinded his son the Cyclops, and Greek attitudes to the sea generally were closer to fear than affection - the sea was where you went to make money and where you drowned.

The founding myth of Athens has him losing a contest. He and Athena competed for the city's patronage: he struck the rock of the Acropolis and produced a spring of salt water, and she produced an olive tree. The Athenians chose the olive, which fed them, gave them oil and gave them their most valuable export - a myth in which a city explicitly chooses agriculture and trade over naval power, told by a people who then built the largest navy in Greece.