A Siren isn't really a sea monster but sailors have told tales of these beautiful women that sing to you and seduce you into their trap of bloodlust. They are said to eat the sailors alive or drowned them deep in the ocean.
Ceto, goddess of the dangers of the ocean and of sea monsters. The flower of the rivers by SARYTH (deviantART)- Pinned by The Mystic's Emporium on Easy goddess Ceto, believed to be the mother of Medusa
The Leviathan melvillei, discovered in 2010 and named after Herman Melville for his novel, Moby Dick. Depicted preying on a baleen whale, it was comparable in size to a modern sperm whale and had fearsome teeth.