In the modern (post-WWII) history of “sea serpent” reports and claims - and there are still reports, albeit rarely - we have only one report of involving human fatalities. This story appeared in the May 1965 issue of Fate Magazine.
In a first-person account, Edward Brian McCleary claimed to have had a terrifying experience on March 24, 1962 off Pensacola, Florida. McCleary and four friends paddled a life raft out to dive on a wrecked ship. A sudden storm came up, forcing them away from land. At night, a fog closed in on them. In the fog, they hear something moving, and then saw what looked momentarily like a “like a telephone pole about ten feet high with a bulb on top” in the fog. The object was, however, a plesiosaur-like animal. More specifically, “The neck was about 12 feet long, brownish-green and smooth looking. The head was like that of a sea-turtle, except more elongated with teeth. There appeared to be what looked like a dorsal fin when it dove under for the last time. Also, as best I am able to recall, the eyes were green with oval pupils.”
This creature proceeded to kill McCleary’s companions one by one. McCleary alone managed to make it to a protruding mast of the wreck they were diving (the U.S.S. Massachusetts), where he clung until daylight.
The Massachusetts sits today in only 26 feet of water in the Fort Pickens State Aquatic Preserve, with portions of the ship still protruding from the sea. McCleary still lives in Florida, though he apparently has not spoken on the subject of the attack since his article came out. He did report the deaths at the time, says the authorities and reporters told him to leave out the sea monster. One body was recovered. The man had died by drowning.
What are we to make of this? If, as some crypto-researchers (myself included) believe, there is at least one large unclassified marine creature behind sea serpent stories, then it would not be surprising if a specimen occasionally took a man in the water, even if humans were not normally its prey. It happens with sharks, as we all know. All we have as evidence is McCleary’s account.
The plesiosaur-like creature striking its victims from the fog sounds like a scene from a bad horror movie, but then so does a shark attack. The very plesiosaur-like sketch McCleary made of his creature shows the head joined to the neck at an odd 90-degree angle (not an impossible angle, perhaps, but one has to reach all the way to a giraffe, not a marine reptile, before one comes up with a real match). It’s a very troubling detail that McCleary does not explain by what light he saw enough to his creature to describe it.
What is, just for a moment, we take the tale as factual? It's fair to say some degree of observer error is to be expected. If we take his sketch as a general, not an exact, representation, than some other reports might be of the same animal. (The 1893 report of the steamship Umfuli comes to mind.) Reports from the Gulf of Mexico are rare, although an online source reports the story of Ray Angerman, whose church youth group saw a similar animal from a bridge near Panama City.
A sea creature report from this area which still interest me was made by naturalist/writer Thomas Helm in 1943. However, Helm described a mammal which does not resemble McCleary’s sketch at all. (Bernard Heuvelmans classified this as an example of his “Merhorse” type, while the Umfuli’s was a “Long-Necked." )
As so often happens in cryptozoology, we are left with a story with no corroborating evidence. That story, as unbelievable as it sounds, still could be true. But at best, we have a whole lot of "not quite impossible"s. Until and unless we get a specimen of a creature that matches McCleary’s beast, though, we must write this off as "likely a fabrication," but we landlocked humans all love a mystery of the sea.
Bibliography Helm, Thomas. Monsters of the Deep. New York: Dodd, Mead, & Co. 1962. Heuvelmans, Bernard. In the Wake of the Sea-Serpents. New York: Hill & Wang, 1968. McCleary, Edward Brian. “My Escape From a Sea Monster,” FATE, May 1965. Online sources including trueauthority.com, unexplained-mysteries.com, answers.yahoo.com, and (for the Ray Angerman story)