The Sea Monster is an aquatic creature in the Goosebumps book Deep Trouble.
At the end of the story, Billy runs into the monster again while looking for The Mermaid.
It is a huge blue monster with one eye, tentacles like an octopus, and has rows of sharp teeth.
The Sea Monster was originally going to appear in the Goosebumps Movie due to appearing in concept art, but it was replaced by E. Ville Creeper's Plants from You're Plant Food!. In the concept art, he has a dark green tone, four red eyes, no-visible mouth, and eight octopus-like tentacles.