I used to be terrified of going into the "deep waters" of the ocean. My family and friends would always egg me on to out to the point where it was like 15 feet deep. I once got stuck with 10 or so jelly fish, just floating helpless out there getting stung over and over. One time in Nags Head (or Hilton Head) a dorsal fin emerged about 5 feet from where I was floating and I think I shrieked like a little girl. It turned out to be a dolphin, but still terrifying nonetheless.
I think deep down I'm still deeply afraid when I go out that far, but I somehow manage to just repress it and think about something else.