Wow, some harrowing experiences in this thread!
Mine was probably also in a swimming pool, we had swimming lessons once a week in elementary school, I never liked it since I was a rather poor swimmer and didn't like water much ever since an accident in a lake when I was 2-3 years old. Anyway, I did technically know how to swim, but I never liked being on the deep end where my feet couldn't touch the bottom if I had to stand up, so I avoided it.
As a sort of game in the pool, the "cool kids" used to swim underneath the ladder/stairs we used to climb into and out of the pool, and it was on the shallow end and felt pretty safe even for someone who didn't like being underwater much.
I decided to give it a go, I guess some of the other kids saw what I was up to and came around to watch, some of these were the very same ones who bullied me from age 7 to around 12, and they would turn out to have some less than friendly intentions.
You see, the steps were loose, they were quite heavy though and simply rested on the edge via a platform at the top, as I swam underneath it, under no more than 3 feet of water or so, some kid(s), don't know exactly who, kicked loose the steps so they fell on me, trapping me at the bottom under what was probably at least 100 pounds of metal, I was pinned to the bottom, stomach and face down and unable to move.
The teacher jumped in and lifted the stairs out of the way, I was later told that I was pinned for over 1:30 minutes and my lungs were burning when I surfaced again.
After this, I never got into a swimming pool with those kids again, and I have been in a pool only twice after this is my entire life (this is now 21 years ago).
The worst part? The kids who kicked the steps onto my back were never so much as reprimanded for doing it; they were the children of "good parents" and I was, unfortunately, the child of some "not so good parents", all about that last name where I come from...