Source : Webster's Revised Unabridged Dictionary (1913)

Sea serpent \Sea" ser`pent\
   1. (Zo["o]l.) Any marine snake. See {Sea snake}.

   2. (Zo["o]l.) A large marine animal of unknown nature, often
      reported to have been seen at sea, but never yet captured.

   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.

Source : WordNet®

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