Giant...things, in the ocean.
In a realistic sense, there is no greater fear to me than tidal waves. Like. Hands down, my #1 fear.
I don't care whether something like THIS
could ever actually happen or not, the imagination and disaster movies sure as heck make it a scary thing to think about, and when you DO hear about REAL events like the Japan tsunami happening, it just makes them even scarier (Though fun fact, I LOVE disaster movies, no matter how bad they are).
In a less realistic sense... I'm not saying I believe in sea monsters,
but sea monsters.
Am I the only one who doesn't find big Godzilla type monsters that scary, but the second they're in the water, they're terrifying? Maybe it's the idea you wouldn't see them coming like you would with Godzilla stomping down the street.