Used to think it was sharks since I saw Jaws when I was maybe five years old and it scared the living shit out of me. I don't think I've even really gone more than ankle deep into the ocean ever since.
But the truth is sharks typically don't go around just looking to attack humans. The only reason you hear about them is because the attacks are so rare. They aren't these psycho eating machines that the movie Jaws made them out to be. I wouldn't exactly be having the time of my life if I found myself swimming next to a shark, but I don't think I'd be quite as freaked out as when I was younger. Crocs and alligators, though...that's a different story. They eat just about anything they can find, and once you're in their jaws, you're done. I'd probably carry a gun with me at all times if I lived in Florida or Louisiana...
Lately, something has been scaring the crap out of me about rattlesnakes. Their bites are lightning quick and potentially fatal...and I doubt I'd just happen to have some anti-venom handy if I did get bit. I can't imagine living in a place like Arizona where some residents see rattlesnakes just sitting on their porch or driveway...Jesus Christ, with my luck, I'd step on one in the dark. Although I guess they've been appearing in Malibu lately as well...oh, joy...