On vacation at the beach?? Step 1: Turn off computer!
Anyway - The Kraken!
Godzilla, hands down. I've loved Godzilla since I was a kid. Just a giant monster that dwells in the depths of the sea that wakes at the sign of human arrogance and destroys everything.
This as well as Sirens even though they might not be from the sea. But they do sing beautifully
The Sirens of Greek mythology were originally bird-like creatures, though they still frequently worked to lure sailors to the doom. So while they didn't live under the ocean, they were still by-and-large seabound. Also, unless you're talking about the jellyfish, Medusa was a very landbound gal.
Well, the general story is that Poseidon copulated with Medusa in a temple to Athena, who punished the woman for defiling her temple by turning her into a gorgon. Of course, the story can vary from place to place. Some say Medusa was wooed by Poseidon, some say she seduced him, and others claim that he [censored] her.
The Greeks worshipped a bunch of jerkwads.
My initial response would be the Kraken! But, the more I think about it, I think I'm much more partial to the Leviathan. The great Sea Serpent, hundred/s of feet in length, able to crush multiple ships in the coils of it's body, and can move incredibly fast through the water. Something about it terrifies me yet excites me at the same time.
Swamp Thing.
Plus a special mention for the Killer Whale in Futurama (Episode: 300 Big Ones).