Sea-Serpents are a type of legendary creature that while not always depicted as malevolent were often used in legends, myths and tall tales as a sort of dragon parallel - Sea-Serpents also had a connection with the famous Kraken.
Much like the Kraken Sea-Serpents were sometimes blamed by ancient sailors for the destruction and sinking of vessels at sea and fantastical tales of battles against gigantic Sea-Serpents make up the culture of some ancient people (notable examples would be the Midgard Serpent of Norse mythology and (arguably) some of the Babylonian gods).
Sea-Serpents have also been classed as a cryptid since science can not tell for certain whether or not such creatures truly exist - though most scientists would agree the legends about man-eating, ship-destroying Serpents is myth.
There are some species of fish, eel and even sea-snakes that can grow incredibly large and these may of given rise to legends of the Sea-Serpent: though like most monsters of the deep there are still a few who believe Sea-Serpents are very much real and not simply mistaken identities.
Sea-Serpents are one of the world's most recurring legends and have appeared in almost every culture imaginable - they can vary from being relatively harmless wonders of the ocean, guardians of hidden treasure or malevolent demons that send men to their watery graves.