In the 19th century there were several major sea serpent sightings on the Gloucester and Maine coasts of New England, which spawned a rather silly mix-up. On August 18, 1817, a meeting of the New England Linnaean Society went so far as to give a deformed terrestrial snake the name Scoliophis atlanticus (thinking it was the juvenile form of a sea serpent that had recently been seen nearby). After the Linnaean Society's misidentification was discovered, it was frequently cited by debunkers as evidence that the creature did not exist; when in fact, the only thing proven by the incident was that the Society had made an embarrassing public error.

A particularly famous sea serpent sighting was made by the men and officers of HMS Daedalus in August, 1848 during a voyage to Saint Helena in the South Atlantic; the creature they saw, some 60 feet long, held a peculiar maned head above the water. The sighting caused quite a stir in the London papers, and Sir Richard Owen, the famous English biologist, proclaimed the beast an elephant seal. The Daedalus' serpent has since been classified as a Super Eel or Type 2C animal. Another skeptical suggestion for the sighting proposed that it was actually an upside down canoe, or a posing giant squid.

Oarfish that washed ashore on a Bermuda beach in 1860. The animal was 16 ft long and was originally described as a sea serpent.

Another sighting took place in 1905 off the coast of Brazil. The crew of the Valhalla and two naturalists, Michael J. Nicoll and E. G. B. Meade-Waldo, saw a long-necked, turtle headed creature, with a large dorsal fin. Based on its dorsal fin and the shape of its head, some experts (such as Heuvelmans) have suggested that the animal was some sort of marine mammal. It has since been classified as a Super Eel (without good justification) or Type 4B. A skeptical suggestion is that the sighting was of a posing giant squid, but this is hard to accept given that squids do not swim with their fins or arms protruding from the water.

More serious sightings from the same place and time have since been classified as Many-Humped Serpents, Classic Sea Serpents, monsters of the deep, or Type 3 animals (see classification systems). Skeptical suggestions for the sightings include giant squid, misidentified snakes, or a wave phenomenon.

On April 25, 1977, the Japanese trawler Zuiyo Maru, sailing east of Christchurch, New Zealand, caught a strange, unknown creature in the trawl. Photographs and tissue specimens were taken. While initially identified as a prehistoric plesiosaur, analysis later indicated that the body was most likely the carcass of a basking shark.

What makes sea serpent sightings different from sightings of other unconfirmed creatures is not only the sheer number of sightings, but the fact that many of these sightings have been made by multiple people at once for sustained periods of time. Instead of just one to a few witnesses purporting to have witnessed the creature for just a few moments, some sightings involve dozens, or even hundreds, of people claiming to have seen the creature over the course of a couple to a few hours. In many cases, these sightings are well documented in the newspapers of the day, although most occurred before any kind of photographic technology existed.

Sea serpent sightings continue today, with reports coming in from the Pacific Northwest and California; the most notable of recent occurrences is the alleged filming of sea serpents by the brothers Bill and Bob Clark in San Francisco bay. In October 2004, the giant squid, long associated with sea monsters and perhaps the source of many mistaken sightings, was for the first time caught on video off the Bonin Islands, revealing for the first time the appearance in life of this "cousin" of sea serpents.