One topic that never fails to intigue cryptozoologists is that of large unidentified marine animals, for which we are pretty much stuck with the name "sea serpent" even though no one thinks there are giant snakes involved. A logical question is why we have no carcasses of such animals. There are large animals, mainly beaked whales, known from a very few strandings, but shouldn't we have at least one definite set of remains if there's a plesiosaur or giant long-necked seal or whatever at large? Reported sea serpent carcasses have either been identified as known animals (cetaceans or basking sharks) or have disappeared before being examined by a scientific authority.
One of the carcass reports that continually resurfaces, so to speak, in cryptozoology is the one netted on April 25, 1977, off New Zealand by the Japanese fishing boat Zuiyo Maru. The surviving photographs show something that does look like a plesiosaur - but also like a decaying basking shark. In this case, the carcass was not kept, but there are the photographs and, even more important, tissue samples.
My view: this is a case we should dismiss.
One online paper, John Goertzen's, claims it cant't be a shark because it has small upper fins above the pectorals. That's not what is looks like to me, though - it just looks like the remains of a dorsal fin is visible on the near side - and no animal in all history, of any type, had such fins. There are a number of anatomical reasons why this is not a plesiosaur, one being that the ribs are far too short. While the overall shape looks somewhat similar to a plesiosaur, again, all rotting basking sharks do. The tissue samples were given to the boat's owner, the Taiyo Fish Company, whose biochemist said the samples were shark tissue. The samples contained elastoidin, which exists only in sharks and rays.
In the interests of debate, here's a Video in which one expert argues for an "unknown" identity (although the reasoning is not clear to me). But I think this paper by Gary Kuban is the definitive one on the subject.
Finally, a comment by Dr. Darren Naish, a British paleobiologist who is not by any means close-minded on cryptozoological subjects, posting in response to an arugment by cryptozoologist Scott Mardis: "It is a rotting basking shark. Yasuda's assertions are irrelevant and his 1978 paper is based on allusions to gross aspects of anatomy that aren't useful in working out the identity of the carcass: you >cannot< look at a very obviously rotting carcass and assume that it represents the original, genuine body shape. Given the strength of the shark hypothesis, I do find it misleading to keep implying that the true identity of the carcass is potentially up for grabs..."
The bottom line: I wish this was a plesiosaur, but I'll bet my house it wasn't.