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 and 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.