sea serpent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A large snakelike marine animal often reported by mariners since antiquity but never positively identified.

from The Century Dictionary.

• noun A name of Ophichthys gomesii, an eel of the family Ophichthyidæ, found in the West Indies.
• noun An enormous marine animal of serpentine form, said to have been repeatedly seen at sea.
• noun In herpetology, a general name of the marine venomous serpents or seasnakes of the family Hydrophidæ.
• noun A chain of salps linked together.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) Any marine snake. See sea snake.
• (Zoöl.) A large marine animal of unknown nature, often reported to have been seen at sea, but never yet captured.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any unidentified sea monster, especially one resembling a snake or dragon.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun huge creature of the sea resembling a snake or dragon