whales, sharks and other really big things in deep, deep water... it's not so much that I'll never stand a chance against one of those animals, should it decide to rip me in half, it's just... big things. in water.
I love to swim, but I'm really uncomfortable when I can't see the bottom due to murky waters or seaweed. I'm alright on ferries, but I'll imagine I wouldn't care much to be on a smaller boat out on the open ocean.
...I got really worked up when I did that thing in Fallout (New Vegas, I guess?) where you raise the plane from the lake, I was all like "gawd,I-hope-it's-not-a-big-plane,-it'll-scare-me-with-its-silhouette D:. *pant-pant*"