In his fascinating book  about the sea monsters that adorn various early maps, Chet Van Duzer writes:
I would suggest that sea monsters on maps have two main roles. First, they may serve as graphic records of literature about sea monsters, indications of possible dangers to sailors – and datapoints in the geography of the marvellous. Second, they may function as decorative elements which enliven the image of the world, suggesting in a general way that the sea can be dangerous, but more emphatically indicating and drawing attention to the vitality of the oceans and the variety of creatures in the world, and to the cartographers artistic talents.. ..During the Renaissance, sea monsters on maps reflect and express an increased general interest in wonders and marvels.. ..the more exotic the creature depicted, the better, and the study of both maps and images of exotic creatures was thought to sharpen the intellect and be educational. Maps decorated with sea monsters, by implication, would offer the viewer an even richer intellectual experience.
If that is the case, the map produced by Olaus Magnus (1490-1557), and shown below, must have been of great significance. Of the many sea monsters depicted, perhaps the most well-known is the sea serpent described by Olaus as follows :
Those who do their work aboard ship off the shores of Norway, either in trading or fishing, give unanimous testimony to something utterly astounding: a serpent of gigantic bulk, at least two hundred feet long, and twenty feet thick, frequents the cliffs and hollows of the sea coast near Bergen. It leaves its caves in order to devour calves, sheep, and pigs, though only during the bright summer nights, or swims through the sea to batten on octopus, lobsters, and other crustaceans. It has hairs eighteen inches long hanging from its neck, sharp, black scales, and flaming red eyes. It assaults ships, rearing itself on high like a pillar, seizes men and devours them.
Such a creature (the red “snake” attacking a ship in the image below) would deter fisherman, and other sailors, from entering Scandinavian waters and, as Van Duzer points out, this may be one of the reasons for the location of sea serpents, and other monsters, on maps.
Sea serpent myths have a wide distribution and examples have been described by Aristotle, Pliny the Elder and many others . Although there were sightings of serpents from Norwegian waters in the Nineteenth Century, the most well-known observation of one of the creatures was made by Captain Peter M’Quhæ of H.M.S.Daedalus off the south-west coast of Africa. As an Officer in the Royal Navy he was viewed as a reliable source and The Times of 9th October 1848 carried a report of M’Quhæ’s sighting. His letter to The Admiralty was quoted by Philip Henry Gosse in The Romance of Natural History  and it contains the following description of the creature:
..it was discovered to be an enormous serpent, with head and shoulders kept about four feet constantly above the surface of the sea, and, as nearly as we could approximate by comparing it with the length of what our maintopsail-yard would shew in the water, there was at the very least sixty feet of the animal [at the surface of the water], no portion of which was, to our perception, used in propelling it through the water, either by vertical or horizontal undulation..
..The diameter of the serpent was about fifteen or sixteen inches behind the head, which was, without any doubt, that of a snake; and it was never, during the twenty minutes that it continued in sight of our glasses, once below the surface of the water; its colour a dark brown, with yellowish white about the throat. It had no fins, but something like the mane of a horse, or rather a bunch of sea-weed, washed about its back..
There are differences to the sea serpent described by Olaus Magnus but, as with all descriptions of these creatures, it is large and snake-like. Gosse, who was a literalist Christian and believer in Creation, had sufficient faith in M’Quhæ’s sighting to propose that the sea serpent was an enaliosaur; spending many pages of The Romance of Natural History sifting through the conclusions of others.
What did M’Quhæ, and others aboard his ship, see? Given that humpback whales using bubble nets could be the origin of belief in the kraken , the answer may lie with the behaviours of contemporary marine animals, although there is no obvious explanation for the appearance of the sea serpent. There are many who still wish to believe in the survival of dinosaurs, and the well-established mythological importance of snakes and dragons with extraordinary powers certainly influences our imagination.
 Chet Van Duzer (2013) Sea Monsters on Medieval and Renaissance Maps. London, The British Library.
 Joseph Nigg (2013) Sea Monsters: The Lore and Legacy of Olaus Magnus’s Marine Map. Lewes, Ivy Press.
 Philip Henry Gosse (1860) The Romance of Natural History. London, J. Nisbet and Co.
The copy of Olaus Magnus's map used in the illustrations above come from Nigg's book (see reference ).