Deep water. Not so much drowning since I'm a good swimmer.. but getting caught in a current or some big scary creature coming up to eat me. Murky/dirty water too because I can't see. And cave diving. I would like to try it... but something about no air being above you gets me. 
I would rather walk through a barn filled with snakes and cobwebs grabbing at my face than swim across a rushing, muddy river more than 10 feet deep. 
I'll go tubing on BIG lakes behind a boat. But you better turn around quick to get me if I bounce off! The longer I have to tread water in the center of a very big and very deep lake, the more panicky I get about what could be swimming around eying my toes. 
If I go snorkeling I can't look out "into the deep" because I'll envision a big shark coming out of it with it's mouth open.  I need to see bottom when I look down, and I can't look side to side unless there is something there besides water. And it better not be a big fish or worse! Dolphins are cool. But I worry about sharks, jellyfish, sting rays, ect. 
But I love water, boats, wading out into the ocean, snorkeling in the shallows.