The Greeks and Romans were great seafarers so it would make sense they had a lot of Sea Deities. I have also included water and river Deities in this list.
Aegaeon A God of violent storms at sea
Amphitrite (Salacia) Sea Goddess and consort of Poseidon
Anapos Local Sea God who resided in eastern Sicily
Aphros Sea God with the body of a horse and tail of a fish (Ichthyocentaurs)
Brizo Goddess of Sailors
Bythos Sea God with the body of a horse and tail of a fish (Ichthyocentaurs)
Ceto Goddess of Dangerous Waters as well as Sea Monsters
Cymopoleia Goddess of Storm Waves and daughter of Poseidon
Doris Goddess of the Bounty of the Sea
Eidothea Sea Goddess and daughter of Proteus
Electra Oceanid and Sea Goddess
Eurybia Goddess of the Mastery of the Sea
Fontus God of Wells and Fresh Water
Galene Calm Sea Goddess
Glaucus Sea God of Fishermen
Leucothea (also known as Ino) Sea Goddess protector of sailors
Nerites Young Sea God
Nereus Old Sea God (The Old Man of the Sea)
Nymphs These young Sea Goddesses number in the many and consist of the Naiades, Nereides and Oceanides.
Oceanus Sea God personification of the oceans
Pan Nature God who is also God of Fishing
Palaemon Young Sea God
Phorcys Sea God of danger
Pontus Early Sea God
Poseidon (Neptune) Main Sea God and God of Horses
Potamoi Sea Gods who are fathers of the many Sea Nymphs
Proteus Early Sea God
Tethys Titan Sea Goddess
Thalassa Early Sea Goddess
Thaumus Sea God
Thetis Sea Goddess
Triteia Sea Goddess and daughter of Neptune
Triton Sea God and son of Neptune
Volturnus God of Waters
Names in parentheses are Roman equivalents.
Over 33 Sea Gods and Goddesses or others related to the sea. I can see why as the Sea, Ocean, Rivers and Lakes provided food and water for living. Some were local in nature and others existed in the stories told. Several were quite important although all had their followings.