According to the Oxford dictionary, the word ocean means "a very large expanse of sea, in particular each of the main areas into which the sea is divided geographically".
According to the Etymology dictionary, the word ocean comes from the Greek god Oceanus "occean (12c.), from L. oceanus, from Gk. okeanos, the great river or sea surrounding the disk of the Earth (as opposed to the Mediterranean), of unknown origin. Personified as Oceanus, son of Uranus and Gaia and husband of Tethys".
Wikipedia says that "Oceanus was a titan, and he was the protector of the greek ships, he have a sister called Thetys, and they created the nymps sea, also known as the three-thousand Oceanids".
There are other derivates of the word, such as Oceania
This word is used in this article.