Some have argued a history of "monster" sightings in the loch provides circumstantial evidence supporting the creature's existence. Note that the validity and origins of these stories have been challenged, along with any "history" predating the early 1930s.

In the early 1970s, a group led by American patent lawyer Dr. Robert Rines obtained some underwater photographs. One was a vague image, perhaps of a rhomboid flipper (others have argued the object could be air bubbles or a fish fin). On the basis of this photograph, Sir Peter Scott, one of Britain's best-known naturalists, announced in 1975 that the scientific name of the monster would henceforth be Nessiteras rhombopteryx1 (Greek for "The Ness monster with diamond-shaped fin"). This would enable Nessie to be added to a British register of officially protected wildlife . It has been noted by London newspapers that Nessiteras rhombopteryx is an anagram of "monster hoax by Sir Peter S." Monster-hunter Dr. Robert Rines replied that the letters could also be rearranged to spell "Yes, both pix are monsters--R."

The underwater photos were reportedly obtained by painstakingly scouring the loch's depths with sonar, over the course of days, for unusual underwater activity. An underwater camera with an affixed, high-powered light (necessary for penetrating Loch Ness' famed murk) was then deployed to record images from below the surface. Several of the resulting photographs, despite their obviously murky quality, did indeed seem to show an animal quite resembling a plesiosaur in various positions and lightings. A few close-ups of what is alleged to be the creature's diamond-shaped fin were also taken, in different positions, indicating movement. The Dinsdale 16 mm film of 1960, which has also been criticized as having an interpretation that has been greatly expanded from the original JARIC report, and numerous sonar contacts.

Perhaps typical of the many unsatisfactory facts about Nessie is the alleged sighting of October 1871. In this incident, "D. Mackenzie" supposedly described seeing something that moved slowly before moving off at a faster speed. People who saw "the monster" were said to describe it as having a hump (sometimes more than one) that looked like an upturned boat. However, although this story has been repeated in several places , no original 1871 source has been cited, casting doubt on the report.

The famed "Surgeon's Photo" (pictured top) was confirmed a hoax, based on the deathbed confessions of Chris Spurling, son-in-law of Marmaduke Wetherell. Spurling claimed the photo, which inspired much popular interest in the monster, was actually a staged photograph of clay attached to a toy submarine. Also notable are the ripples on the photo, which fit the size and circular pattern of small ripples when photographed up close, not large waves. Wetherell, a big game hunter, had been tricked into searching for an imaginary monster around the loch based on evidence which turned out to be the result of children's prank. He was publicly ridiculed in the Daily Mail, the journal which employed him. To get revenge, Marmaduke Wetherell set this hoax up, with the help of Chris Spurling (his son-in-law as mentioned), who was a specialist in sculpture, Ian Marmaduke (his son), who bought the material for the fake Nessie, and Maurice Chambers (an insurance agent), who was to call and ask Robert Wilson (a surgeon) to show the pictures. Well before Spurling's claims, however, others had argued the photo was that of an otter or a diving bird. There are in fact two "Surgeon's Photos," which depict slightly different poses, leading some to argue the photos are evidence against a hoax. However, at the time of this confession his father had already died, and Spurling wanted to disprove the photo. The surgeon who was credited for taking the photo never claimed he hadn't taken it either.

The "flipper photograph" above has been highly retouched from the actual original image. The Museum of Hoaxes  shows the original unenhanced photo.

In July 2003, the BBC reported an extensive investigation of Loch Ness by a BBC team, using 600 separate sonar beams, found no trace of any "sea monster" (i.e., any large animal, known or unknown) in the loch. The BBC team concluded that Nessie does not exist.  In 2004, a documentary team for Channel 4 (primarily consisting of special effects experts from movies) deliberately tried to make people believe there was something in the loch. They constructed an elaborate animatronic model. Despite setbacks, it was a success, and numerous sightings were reported on the day, in the places they performed the hoaxes.

In addition, sunlight does not penetrate very deep into the water because of peat washed into the loch from the surrounding hills. This limits the amount of algae in the loch, thereby reducing the number of plankton, small fish, and then large fish up the food chain. The loch simply does not have enough food to support animals as big as a plesiosaur, particularly a breeding population of plesiosaurs. Palaeontologist Neil Clark has also proposed that the 1933 sightings were actually elephants from the travelling circus of Bertram Mills.

According to the Swedish naturalist and author Bengt Sjögren (1980), the present day belief in lake monsters in for example Loch Ness, is associated with the old legends of kelpies. Sjögren claims that the accounts of lake-monsters have changed during history. Older reports often talk about horse-like appearances, but more modern reports often have more reptile and dinosaur-like-appearances, and Bengt Sjögren concludes that the legends of kelpies evolved into the present day legends of lake-monsters where the monsters "changed the appearance" to a more "realistic" and "modern" version since the discovery of dinosaurs and giant aquatic reptiles from the horse-like water-kelpie to a dinosaur-like reptile, often a plesiosaur.