Reported in all the oceans of the world, Sea Serpents have been known from ancient times.  Sea Serpents live in both fresh and salt waters,  and they were the most feared creatures of the ocean with hundreds of reported sightings to inspire terror. 
People often reported sighting during the warmer months, and it is widely believed that Sea Serpents only appear in still, warm weather.  However, reports of Sea Serpents in particularly cold waters and during storms indicate these creatures may appear at any time.
Sea Serpent by Dee Dreslough.
Sea Serpents range in size drastically, from 30 feet to 300 feet (9 m - 90 m) long  with bodies ranging from 5 feet to 20 feet (1 m - 6 m) thick.  They range in color from greyish  to chocolate  to black.  Some sea serpents have a mane down their necks, similar to a horse;  others have heads shaped like those of a horse and the mane presented resembles seaweed. 
Some reports claim that sea serpents have a single horn, approximately one foot (31 cm) in length shaped like a marlinspike.  While many sightings of sea serpents claim they have serpentine bodies without limbs that end with a long, steering tail, there are just as many reports of sea serpents with webbed limbs, fins, or even wing-like appendages, likely used for steering in the water. 
While the description of individual Sea Serpents vary, there is one particular attribute that remains the same. They all have large eyes, usually blue or pewter in color. 
Sea Serpents in History and Sightings
Today, many people believe that sailors in the fourteenth century and beyond were afraid of falling off the edge of the world because they believed the earth was flat. If this was true, little evidence of this belief was left behind, at least in comparison to the evidence indicated the fear of Sea Serpents.
Many nautical charts bare the inscription 'HERE BE DRAGONS' to indicate danger or the unknown.  The rampant fear of Sea Serpents increased dramatically in the fifteenth century, as European colonialism spread.  This is likely the result of the countless stories describing a ship attacked by a serpent. 
Sometimes these stories told of a serpent that washed men off the decks into the sea. It did this by surfacing along the side of the ship and spouting a powerful jet of water.  More often, the creature levied a terrifying attack from above, snatching men off the decks with its long neck and powerful jaws.  Some stories even claimed the serpents wrapped their coils around smaller ships, crushing them to timbers. 
The sightings of sea serpents often describe them as scaly, moving with vertical undulation, with the humps of the serpents body rising above the water. 
Sea Serpents as Cryptids
Cryptozoologists are interested in Sea Serpents, owing to the number of sightings; however, many of them speculate that people mistake large squids, pieces of kelp, and even certain species of dinosaurs for Sea Serpents.  For example, the Basilosaurus, or some descendent thereof, could have easily been mistaken as a Sea Serpent. 
The number of Sea Serpent sightings has dropped in recent years, but whether or not this is due to people no longer spotting them or to people being afraid of possible ridicule and therefore not reporting them at all, is up to debate. 
- Ingpen 63
- Nigg [Dragons] 110
- Gould 271
- Gould 282
- Beck, Folklore and the Sea
- Nigg [Dragons] 17
- Gould 290
- Gould 305
For more information on footnotes and references, please see the bibliography.