The Collaborative International Dictionary of English v.0.48:

Sea serpent \Sea" ser"pent\
   1. (Zool.) Any marine snake. See Sea snake.
      [1913 Webster]

   2. (Zool.) A large marine animal of unknown nature, often
      reported to have been seen at sea, but never yet captured.
      [1913 Webster]

   Note: Many accounts of sea serpents are imaginary or
         fictitious; others are greatly exaggerated and
         distorted by incompetent observers; but a number have
         been given by competent and trustworthy persons, which
         indicate that several diverse animals have been called
         sea serpents. Among these are, apparently, several
         large snakelike fishes, as the oar fish, or ribbon fish
         (Regalecus), and huge conger eels. Other accounts
         probably refer to the giant squids (Architeuthis).
         Some of the best accounts seem to describe a marine
         saurian, like the fossil Mosasauri, which were large
         serpentlike creatures with paddles.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sea serpent
    n 1: huge creature of the sea resembling a snake or dragon

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "sea serpent":
   Loch Ness monster, alevin, benthon, benthos, cetacean, dolphin,
   fingerling, fish, fry, game fish, grilse, kipper, man-eater,
   man-eating shark, marine animal, minnow, minny, nekton, panfish,
   plankton, porpoise, salmon, sea monster, sea pig, sea snake, shark,
   smolt, sponge, tropical fish, whale