Sea monsters are sea-dwelling mythical or legendary creatures, often believed to be of immense size.
During their journey to Isla Fortuna, the crew of the Barnacle encountered a sea monster. When the Barnacle stopped in the middle of the sea, the young adventurers thought they have hit a reef, but "the reef" was actually a sea monster.
Along the shores of the Isle of Man in the early 1700s, a sea monster ate all the fish causing a famine throughout the village. This caused young Billy Turner to be sent away to seek riches for his family in North Carolina. It had also destroyed his beloved boat so he had no transportation. On the course of his adventures, Billy found out the sea monster was controlled by Captain David Donovan, a ruthless pirate. After realizing that the eel-like sea monster was the cause for his village's famine, Billy slayed the beast by stabbing it in the heart.
David Donovan's sea monster lived off the wreckage from ships destroyed by Donovan. The sea monster was a long scaly beast who resembled a large eel. Oddly enough, the creature's heart was located in its jaw instead of its chest.