From Greek okeanos, of unknown origin. The Greeks used the word to refer to a great river which was supposed to run around Europe, Africa, and Asia, and its god, Oceanos. He was a Titan, the son of Uranus and Gaia, and the husband of Tethys (the Titan-goddess of the Nile and other mighty rivers). This great river is often associated with monsters of the deep in other mythologies — such as the Norse Jormungand, the Midgard Serpent and son of Loki, an ouroboros encircling the world.
The word was picked up in Latin and used in the phrase mare oceanus, "Ocean Sea" (meaning the Ocean Sea as opposed to, say, the Mediterranean Sea or the Black Sea). This is why, when Columbus crossed the Atlantic, the Spanish named him "Admiral of the Ocean Sea." It reached English through Old French in the 13th century, and began to be used as a moniker for the Atlantic and other large bodies of water in the 14th century.
Spiritually, it begins with wholesome energy, passing through protective turbulence, and is released into grounding.
From Proto Germanic saiwaz, meaning a large body of water. The word is unrelated to any in other Proto Indo European daughter languages, and does not appear to have come from Finnish or the other Baltic tongues. Saiwaz became sæ in Old English, and sea in Middle English. It was first applied to the moon's mares in the 1660's.
Spiritually, sea is a word of sunlight, fertility, and growth, developing into high, tense energy. As such it may more naturally indicate the surface of the water, rather than its depths.
From Proto Indo European dheub, referring to hollowness and depth. In Old Irish, dheub became domun, "world", because of the world's deep foundations. In Proto Germanic, it became deupaz, and in Old English, deop, maintaining both its literal sense and metaphorical senses of profundity, awe, mysteriousness, seriousness, and solemnity.
Spiritually, deep has the same high, tense -ee- sound as sea, and contains the same high, tense energy; but this energy is engaged through a doorway, a decision, and is drawn to a definite location or point. Its phonosemantics is similar to that found in weep, reap, seep, leap, and keep.