Poseidon was the Lord and Ruler of the Sea (Mediterranean) and the Friendly Sea (now the Black Sea). Underground rivers were his too. He had an elder brother at the name of Zeus and his wife was Amphitrite, daughter of Nereus. Before he was a sea god Poseidon and Demeter had a kid named Persephone and a horse named Arion.
Ocean was the Lord of the river Ocean, a great river encircling the earth. Ocean, a Titan, had a wife at the name of Tethys who was a Titan too. The Oceanids, the nymphs of this great river, were his daughters and the gods of all the rivers on Earth were their sons.
Pontus, which means the Deep Sea, was the son of Mother Earth. Pontus was the father of Nereus who was a sea-god far more important than he himself and Pontus was the husband to Mother Earth.
Nereus was called the Old Man of the Sea (Mediterranean). "A trusty and gentle god who thinks kind thoughts and never lies," says Hesiod. Nereus's wife was Doris, a daughter of Ocean. He had fifty lovely daughters who were the nymphs of the Sea.
Triton was the trumpeter of the Sea. His trumpet was a great shell. Triton was the son of Poseidon and Amphritrite.
Proteus was sometimes said to be the son Poseidon and sometimes his attendant. Poteus had both the power of foretelling the future and of changing his shape at will.
The Nadiads were also the water nymphs. They dwelt in brooks and springs and fountains.
Leucothea and her son Palaemon, once mortals, became divinities of the sea , as did also Glaucus, but all three were unimportant.