I'd have to say mine was when I was at the wee age of 8.
My siblings and I were out swimming with some friends and their parents. What I remember mostly was that I got seperated very quickly from the rest of the group and drifted into the deeper end. I found it getting more and more difficult to swim with each passing second. The next thing I know, it was getting harder to keep my head above water, so I started screaming.
Even that became difficult because of the water getting in my mouth. I hadn't been so terrified in my life, and the next thing I knew, I blacked out.
The next thing I know, the father our friends was holding me up and said, "I gotcha!" I've never been so happy to hear someones voice before.
So now, in my twenties, I can swim very well, but I guess that experience is the reason that i'm still afraid to go into parts of pools or other bodies of water that are higher than 8 feet or so. Go figure.
What about some of your experiences? What are some of your scariest?