The Collaborative International Dictionary of English v.0.48:

Sea \Sea\ (s[=e]), n. [OE. see, AS. s[=ae]; akin to D. zee, OS.
   & OHG. s[=e]o, G. see, OFries. se, Dan. s["o], Sw. sj["o],
   Icel. saer, Goth. saiws, and perhaps to L. saevus fierce,
   savage. [root]151a.]
   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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. The ocean; the whole body of the salt water which covers a
      large part of the globe.
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            I marvel how the fishes live in the sea. --Shak.
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            Ambiguous between sea and land
            The river horse and scaly crocodile.  --Milton.
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   4. The swell of the ocean or other body of water in a high
      wind; motion or agitation 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.
      [1913 Webster]

   5. (Jewish Antiq.) A great brazen laver in the temple at
      Jerusalem; -- so called from its size.
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            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.
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   6. Fig.: Anything resembling the sea in vastness; as, a sea
      of glory. --Shak.
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            All the space . . . was one sea of heads.
                                                  --Macaulay.
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   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.
         [1913 Webster]

   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 Beyond 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, to adopt the calling or occupation of a
      sailor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

at sea
    adj 1: perplexed by many conflicting situations or statements;
           filled with bewilderment; "obviously bemused by his
           questions"; "bewildered and confused"; "a cloudy and
           confounded philosopher"; "just a mixed-up kid"; "she felt
           lost on the first day of school" [syn: baffled,
           befuddled, bemused, bewildered, confounded,
           confused, lost, mazed, mixed-up, at sea]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "at sea":
   abashed, abroad, adrift, afloat, astray, bewildered, bothered,
   by sea, by water, clueless, confused, discomposed, disconcerted,
   dismayed, disoriented, distracted, distraught, disturbed,
   embarrassed, guessing, homeward bound, in a fix, in a maze,
   in a pickle, in a scrape, in a stew, in blue water, in soundings,
   lost, making way, mazed, off soundings, off the course,
   off the heading, off the track, perturbed, put-out, turned around,
   under bare poles, under sail, under way, upset, with sails spread,
   with way on, without a clue