There are four mythical sea monsters that I have found interesting.
Leviathan was a sea monster described in the Bible as being the gate keeper of Hell. It also means whale in modern Hebrew and it can also be described as being a giant fish or sea serpent capable of destroying large ships.
The Sea Orm is similar to sea serpents only they are not reptilian but more closely resembling giant worm like monsters, to which its alias name also refers to as worm or vurm. Its name is often used in both British and Scandinavian mythology.
Jormungand also called the Midgard Serpent from Norse Mythology was cast out into the sea by Odin, by which he grew so large it was able to circle the Earth and bite its own tail. The Midgard Serpent would do battle with Thor by which it was defeated and killed only to also kill Thor in the process with its poison.
A European Mythical sea monster which resembles a whale like creature and capable of destroying large boats. Its name is Latin which means Whale.
Out of these monsters I quite like the worm like Sea Orm and the Leviathan.