The Collaborative International Dictionary
Sea monster
Sea monster \Sea" mon"ster\ (Zo["o]l.) Any large sea animal.
Wiktionary
sea monster
n. (context mythology English) A large and aggressive creature living in or under the sea that attacks ships.
Usage examples of "sea monster".
He knew now that he had not lost his mind in that instant when he had seen the sea monster advance into sinking Carterstown under direction.