'Sea serpent' definitions:

Definition of 'sea serpent'

noun

Huge creature of the sea resembling a snake or dragon

From: GCIDE

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]