Paleobiologist Darren Naish, one of the most thoughtful of scientifically-trained cryptozoologists, here takes a detailed look at one of the semi-holy grails for those who believe in large unidentified animals still prowl the seas.
Naish agrees the eyewitness evidence for some sort of elongated large marine animal is impressive, but he can't accept one of the most-discussed pieces of physical evidence, the Naden Harbor, British Columbia, carcass of 1937. Naish wonders if this 3-4 meter, very slender, odd-looking thing did, as the contemporary reports had it, come from the gullet of a sperm whale. Ed Bousfield and Paul LeBLond published a controversial paper naming this the type specimen of a marine reptile, Cadborosaurus willsi. Naish agrees he does not know what this thing was (the specimen was lost, and only photographs remain), but is quite sure that Bousfield and LeBlond entered into far too much speculation given the limited amount of data one can be sure of from the photographs.
COMMENT: While the whole topic is often buried in the silly-season term "sea serpent," there really is a suprisingly good body of sightings that remain unexplained. The gold standard, as Naish notes, is the 1905 sighting by two well-qualified British naturalists on the yacht Valhalla, who carefully observed and sketched an animal that still cannot be reasonably assigned to any known species. More details are available in (of course) my book Shadows of Existence, among other sources.