The photographic record for "sea serpents" - that is, photos, movies, or videos which show clearly undescribed species of large animals - is not good. Arguably it's non-existent (and no one is sadder about that than I am).
The first "sea serpent" photo is dated 1908, supposedly from the San Francisco Examiner, although some people who have looked can't find the issue. We do have the picture, which was republished in 1933 in a less-cropped version. The object looks weird, sort of a big black arched thing that might not even be part of the same original photograph. Smithsonian herpetologist George Zug, who was open to cryptozoological claims, thought he could see a tow rope at one end.
Sea serpents assiduously avoided cameras after that, which is a mark (not a damning one, as there are few or no photos of some beaked whales at sea, but an important one) against their existence.
(The 1908 photo is shown here: as far as I can tell, copyright has expired)
The "Mary F" photographs allegedly taken off Cornwall in 1976 by a semi-anonymous person made for some press, but the case is not even a good hoax. The "animal" looks like nothing that ever lived and sits absurdly high in the water.
The videotape of "Chessie" in Chesapeake Bay (1982) is more intriguing. It shows a dark animate object low in the water, and it's not immediately identifiable as anything known. Zug thought it was definitely animate but not clear enough and close enough to say for sure it was an unknown animal, either. A swimming python or anaconda has been suggested, though what it was doing in the bay is a good question. All we can say is that it MAY show a new species.
There are a variety of "carcass" photographs, but they are universally identified as known animals (usually basking sharks, though some look like cetaceans). I'm not aware of a single exception unless perhaps it's the Naden Harbor carcass photograph of 1937, of an animal recovered from a sperm whale's stomach. It was even used as the basis for a formal species description (Cadborosaurus willsi) (which technically isn't valid: you have to have a physical type specimen, not a photo, and no one knows what happened to the original.). Also, it's subject to counter-arguments that it was a shark. Given that's I'm not an expert in marine anatomy, I will only say that I can't take one look and say "shark" the way even I very often can. It bothers me how well the spinal column has held up inside the digestive efforts of the whale, and he head looks odd. I could be wrong on both points, though, so I'll admit this may be just a peculiar shark carcass.
At 12 feet or so, it's well within shark size: I wrote in one of my books that sperm whales were not known to prey on basking sharks, but Richard Ellis in his book The Great Sperm Whale mentions a case of 14-foot basking shark taken from a whale's stomach. (Philip Hoare, in his book The Whale, says an intact 30-foot shark was recovered from a sperm whale, but I don't see how that's even possible. I suspect a misprint or mistranslation of measurements.)
The only closeup photos offered for a sea serpent, shot by Robert le Serrac in 1965, are distrusted by everyone: the photographer all but admitted to a hoax, and the photos likely show an inanimate object either discovered or deliberately manufactured for the occasion.
This video from Norway is kind of interesting, but it is shot in a lake, and I'm going to confine myself to sea animals today: I mention it because the term "sea serpent" is used a lot in media mentions.
This new item from Ireland (allegedly from an arm of the sea called Lough Foyle) originally struck me as a towed object, although I suppose a whale isn't out of the question - there's hardly a minute of footage, so a cetacean might not blow or show flukes in that span. A reporter, though, has matched the background to a site 130 miles from the alleged sighting - a huge red flag.
And that, frankly, is about it. One disputed photograh, one intriguing but not definitive videotape, and that's it. For an animal reported since before photography began (way before) that's a pretty paltry record.
Have I missed any cases?