Sea serpent
Sea serpent has come to mean almost invariably a mythical sea monster that is generally long and serpentine. There are occasional cases reported of long serpentine creatures in the ocean, sometimes even multiple person sightings, especially on ships or at the beach.In the 1800s, the eastern coast of the United States and New England had several major sightings and at the August 18, 1817 meeting of the New England Linnaean Society they even gave it the name Scoliophis Atlanticus. Nevertheless, after some hoaxes it was debunked.
They are generally categorized as someone misinterpereting a real animal, such as the sea snake.