The great sea serpent is featured in ancient legends and folklore going back to the earliest origins of written literature. However, the serpent really came into widespread public interest in the great age of sail, especially from the 16th to early 19th Centuries. Once steamships became routine, reports of the serpents dropped to almost zero and have continued at essentially zero ever since.  There is one very obvious explanation for many if not all of the serpent's appearances.  It was usually seen at a fairly large distance from the ships, and thus was nearly on the horizon.  The sketches loosely based on "eye-witness" sightings were not made by the witnesses and presumably contain huge amounts of artistic license. The serpent is variously depicted as reptilian, with scales, or alternately a thick coat of hair!

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The only place in the world that I know of where sea monsters are still routinely sighted is off the coast of Cornwall, England. Reports are still coming in from there once in a while, involving the sighting of what is usually called "Morgawr."

 

Do you know what you are looking at here? This is a very likely explanation for almost all sea monster sightings.