What does sea mean?
The definition of sea
Sea, n.
Etym: [OE. see, AS. sæ; akin to D. zee, OS. & OHG. seo, G. see, OFries. se, Dan. sö, Sw. sjö, Icel. sær, Goth. saiws, and perhaps to L. saevus firce, savage. sq. root 151 a.]

1 One of the larger bodies of salt water, less than an ocean, found on the earth’s surface; a body of salt water of second rank, generally forming part of, or connecting with, an ocean or a larger sea; as, the Mediterranean Sea; the Sea of Marmora; the North Sea; the Carribean Sea.
2 An inland body of water, esp. if large or if salt or brackish; as, the Caspian Sea; the Sea of Aral; sometimes, a small fresh-water lake; as, the Sea of Galilee.
3 The ocean; the whole body of the salt water which covers a large part of the globe. I marvel how the fishes live in the sea. Shak. Ambiguous between sea and land The river horse and scaly crocodile. Milton.
4 The swell of the ocean or other body of water in a high wind; motion of the water’s surface; also, a single wave; a billow; as, there was a high sea after the storm; the vessel shipped a sea.
5 (Jewish Antiq.)
A great brazen laver in the temple at Jerusalem; — so called from its size. He made a molten sea of ten cubits from brim to brim, round in compass, and five cubits the height thereof. 2 Chron. iv. 2.
6 Fig.: Anything resembling the sea in vastness; as, a sea of glory. Shak. All the space . . . was one sea of heads. Macaulay.

Note: Sea is often used in the composition of words of obvious signification; as, sea-bathed, sea-beaten, sea-bound, sea-bred, sea- circled, sealike, sea-nursed, sea-tossed, sea-walled, sea-worn, and the like. It is also used either adjectively or in combination with substantives; as, sea bird, sea-bird, or seabird, sea acorn, or sea- acorn. At sea, upon the ocean; away from land; figuratively, without landmarks for guidance; lost; at the mercy of circumstances. “To say the old man was at sea would be too feeble an expression.” G. W. Cable — At full sea at the height of flood tide; hence, at the height. “But now God’s mercy was at full sea.” Jer. Taylor.
— Beyond seas, or Beyond the sea or the seas (Law), out of the state, territory, realm, or country. Wharton.
— Half seas over, half drunk. [Colloq.] Spectator.
— Heavy sea, a sea in which the waves run high.
— Long sea, a sea characterized by the uniform and steady motion of long and extensive waves.
— Short sea, a sea in which the waves are short, broken, and irregular, so as to produce a tumbling or jerking motion.
— To go to sea, a adopt the calling or occupation of a sailor.