Poseidon was the Lord of the seas, the son of Cronus and Rhea. Cronus had swallowed him as well but then he brought him up again together with his brothers and sisters. After the Titanomachy and the defeat of Cronus, Poseidon became the King of the Sea and the Islands. He was the top-ranking among the sea-deities, to whom they were subjected.
They used to paint him as an old white-bearded man with fair-white hair and blue eyes, a peaceful look but sometimes a wrathful one, with a band round his head -like Zeus- sometimes naked and sometimes in clothes, carrying a trident in his hand which Cyclopes had given to him before the Titanomachy. The trident was said to symbolize the third kingdom of the Universe which was the Sea, i.e. Poseidon's Kingdom. A huge shell was his carriage drawn by two sea-horses and he was usually accompanied by Glaucus, Palaemon, Thetis, Nereides and Triton who was his "trumpeter".
However, Poseidon should not be strictly connected with the narrow sense of the "Sea" since he captures the driving-force of every phenomenon occurring either at the bottom of it or on its surface: earthquakes, volcanoes, typhoons etc. His ability to agitate the water indicates his connection with the sea-storms as well as with the seismic-waves which are usually caused after an earthquake tremor. Moreover, Poseidon's quarrels with other Olympian gods (Poseidon-Athena, Poseidon-Hera) take place in regions which are in or near seismic zones even today or were such in the past.
Poseidon's attributes and abilities are unfolded before the reader in the Orphic Hymn to Poseidon:
"I'm calling upon thee, Poseidon, the Lord of the Earth and the Sea,
thou who disturb the water and shake the Earth causing plenty of waves...."
In the same hymn Poseidon is said to be one of the oldest gods who dominates over the Earth, second after Zeus.
Apart from Poseidon there is a plethora of other sea-deities: Oceanus, Pontus, Thalassa (Sea) or Tethys, Forkys, Thaumas, Nereus, Nereides, Oceanides, Triton, Proteus, Glaucus etc. This plethora of sea-gods and goddesses indicates how much associated Prehistoric Hellenes were with Water which constituted one of the three fundamental forms of the Universe. We could even say that all these sea-deities were invented by ancient Hellenes in their effort to give a complete and detailed description of Poseidon's attributes i.e. Water-Sea properties.
In the hymn to Oceanus, on the other hand -where Orpheas is calling upon the immortal lord, the everlasting, the greatest among gods and mortal humans, the god who surrounds the Earth and causes the creation of rivers and seas- the separate use of the words "sea" and "oceanus" could mean nothing but the degree of connection between the Sea and Prehistoric Hellenes who seem to know quite well the difference between the sea and the ocean. But could this really have been possible if they had not experienced any voyages ( The Argonautic Expedition) not only across the sea which surrounded them, but also beyond it?