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.
Cthulhu is a malevolent entity who hibernates somewhere beneath the ocean in the South Pacific. The imprisoned Cthulhu is the source of constant anxiety for mankind at a subconscious level, and also the subject of worship by a number of human religions.
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