I do not do open water that I can't stand up in. Water has always made me incredibly tense (not counting showers, doing laundry, or rain, etc.), and I can't even float - I just sink like a rock. I had a near-drowning incident when I was 12 and I just cannot deal with going out on a boat in deep water. No cruise ships for me!! Oddly, bridges over water are fine. I have walked the Golden Gate Bridge many times and love it, but I also understand why some people would never want to do it.
When I was six years old or so, I saw an episode of Space:1999 called "Dragon's Domain" that scared the ever-lovin' snot out of me. Obviously my dad didn't know what this ep had in it at the time or he probably wouldn't have let me see it (at least I would hope not). It gave me nightmares forever after watching it. I know of several people my age who were freaked out by it as small children as well. The visual made enough of an impression, but it was the sound of the creature that really stuck in my head. Definitely NOT the type of thing a little kid should watch when they're still trying to figure out that what isn't real can't actually hurt you. This is just a short idea from youtube of what it was:
[video=youtube;7tbXhu09m5s]http://www.youtube.com/watch?v=7tbXhu09m5s[/video]