Sea Serpent

Sea serpents are a kind of sea monster either wholly or partly serpentine. Sightings have been reported for hundreds of years, and recent work by Bruce Champagne indicates that there have been 1,200 or more all told. Sea serpents have been seen from both ship and shore, and by multiple people at once, groups that sometimes count scientists among their number. Despite the numerous sightings, though, no credible physical evidence has been recorded and it is uncertain whether or not the serpents actually exist.

More info at  en.wikipedia.org