Rhea, and a brother of Zeus. The Doric form of his name was Poteidan, and this is usually understood as meaning 'husband of Da', Da being, perhaps, an old pre-Greek name of the earth goddess who later became Demeter.
He is thus, as god of the sea, complementary in his powers and attributes to the goddess of the land, and one of his best-known by-names, Gaieochos, supports this, since it means 'earth-holding', from gaia, 'earth' and ochos, 'that which holds'.
The sea therefore 'holds' the earth. Those who feel that such an association with 'earth' can be ambiguous, prefer to derive his name from an exclusively 'watery' source, such as pontos, 'the open sea', potos, 'drinking' or potamos, 'river', all linked to the root po which relates to drinking and water (as in English 'potion').