The Sea Monster is the secondary antagonist in the Goosebumps novel Deep Trouble. The Sea Monster is a gigantic many-tentacled octopus with suckers, poisonous tentacles and a fanged mouth. The Sea Monster is first seen when Billy, the protagonist, is diving with his sister Sheena and finds himself being attacked by a giant octopus, whose tentacles reach out to grab the boy and drag him underwater. The Sea Monster eyes him hungrily and tries to drag him down but he is saved by his uncle when he gets to surface who tells him its just seaweed.
Later, Billy goes for another swim and knows it can't be seaweed attacking him because seaweed doesn't have claws. So he screams and struggles but soon he sees the giant head of the Monster break water in a miniature tsunami and it roars at him but he realizes it was just a nightmare.
Billy goes for a morning swim but is almost eaten by a hammerhead shark and saved by a legendary mermaid.
Afterwards, the Sea Monster is not seen at all but reappears in the end, where it breaks the surface when Billy goes off looking for the mermaid. Billy sees the Monster and screams, wondering if anyone would believe him now.