Right, so mine is natural or murky bodies of water. I'm okay in a nice clean blue chlorine-filled pool but outside of that I get nervous.
The weirdest part about this is that it even freaks me out watching it on TV, films, and even videogames. I actually have to get someone else to do any part that involves water in a videogame I'm playing.
I can't honestly figure out why it squicks me out so much but I get physically uneasy when watching on a screen. Think it's something to do with the fact that in water, I wouldn't be able to escape from a situation as quickly as if I were on land, and it's murky so I wouldn't be able to see anything coming. Might also have to do with seeing dead fish float by my goggles once when swimming in a lake. Oddly, I love aquariums, and will swim in an open body of water after awhile without thinking about it once I'm actually in the water.
It's the only fear I have. I'm fine with rodents, the dark, insects, heights, blood, etc. It's just... water.

So, what's your completely stupid pointless fear?