The oarfish

The Regalecus glesne, is a strange eel-like animal rarely seen that has been measured at up to twenty-five feet in length. The oarfish is bright silver in color and has a high, bright red crest of spikes running down the back of its snake-like body. 

In 1930, Dr. Anton Brun caught what he believed to be an eel larva. It was 2 m (6 ft) long and he made the calculation that if it grows like regular eels, it would mature to eighteen times its current size making the adult over 33 m (110 ft) long.  

Two men were gathering seaweed on the coast of Bermuda in 1860 when they came across a serpent-like creature stranded in the rocks. They killed it and the animal was reported as a sea serpent until a naturalist eventually showed up and identified the creature as an oarfish.

The frilled shark

An unusual species of frilled shark, Chlamydoselachus anguineus, might also be taken for a sea serpent. Like the oarfish it is eel-shaped. It has a single dorsal fin placed well back along the body, which can appear as a mane. The frilled shark has an ancient history and is almost a living fossil. It would truly be a likely candidate for a sea serpent if it was only a little larger. So far the largest known frilled shark was only six feet nose-to-tale. In 1880 Captain S. W. Hanna netted a long eel-like shark that measured some twenty-five feet. While not matching the description of Chlamydoselachus anguineus exactly, it is probably a close relative and suggests there may be some giant frilled sharks in the sea that could be taken for sea serpents.

The salt water crocodile

One dangerous candidate as a sea monster is the salt-water crocodile. These creatures, living in the India Ocean and the area around Southeast Asia and Australia have been measured to lengths of 18 feet and weighing almost a ton. Unconfirmed reports indicate they may get as long as thirty feet. They are hungry, aggressive and often attack people.