For reasons that still elude me, the vast majority of people who come here via Google searches come looking for "shark attacks."
Anyway, if that's you you should definitely watch the 2004 film Open Water. Not just for the sharks. In fact, not even primarily for the sharks. But just to have the complete crap scared out of you.
Like the Blair Witch Project, it's a very visceral movie that cuts right into what you know most. No birds pecking at your head, no shower scenes -- just situations you can easily imagine. Which makes it all the worse.