All fishermen Of Norway are agreed that there is a Sea Serpent 200 feet [60m] long and 20 feet [6m] thick that lives in caves and rocks near Bergen ... He hath commonly hair hanging from his neck a cubit [46-56cm/ long, sharp scales, and is black and hath flaming, shining eyes. He puts his head up on high like a pillar.
Erik Pontoppidan, Natural History of Norway (1752-1753)
Reliable sightings of sea serpents are far more common than most people think. For his cryptozoological classic In the Wake of the Sea Serpents (1965) Bernard Heuvelmans collected reports of over 500 clearly documented sightings from the previous two centuries. Most of the beasts were observed by more than one witness, some by over 100. He also studied hundreds of more vague accounts. For each sighting that he recorded one can safely say there are many he did not discover apart from any other reason, claiming to have seen a sea serpent has blighted many lives, so witnesses may advisedly think twice before reporting what they have seen.
Sea Serpent is the term that comes to mind whenever people see a large unknown monster in the ocean. The term has probably been applied to many different creatures but, as with lake monsters, most described sea serpents fall into two main categories: what we might call the 'true' sea serpents, many-humped, serpentine creatures which appear to move with vertical coils; and plesiosaur - like monsters with a large body and a long, slender neck. Sightings are, as Scandinavian sailors have always maintained, commonest in calm weather.
The estimated lengths of sea serpents vary from 20 ft to 250 ft (6 to 75m), but most guesses fall in the middle range, suggesting an average length of 150ft (46m). In fact, in terms of body - mass this is not particularly large for a marine creature: length for length, a sea serpent would, weigh much less than a whale. The largest whale on record was 113 ft (35m) long and weighed over 150 tons. A sea serpent of the same weight would be about 250ft (75m) in length.