Well, it was a joke, but it wasn't successful...
We took a friend's trampoline and tried to sink it in his pool. For some reason it floated 
Air in the piping probally. I remember once when I was about 8 years old, I was at the local pool for our neighborhood. It was the last day of summer, and we had been getting crap from the lifeguards all year. You see, this was pre-pubescent hijinks at it's best, and we had no concept whatsoever of the female form. While they were only 5 years older than us, it felt much older. The man was keeping us down, and something had to be done.
So, we decided to sink the lawn chairs. Unfortuanately, they took much longer to sink than expected. We got caught after successfully sinking one. Somehow that single lawn chair represented all that was gloryful, and while we were overcome, I think we all felt like our small victory was sufficient enough.
That was the greatest summer ever...

EDIT: Err... well my point was this: the piping in a lawn chair and trampoline are similar, and the reason why your plan failed was most likely similar to my problem. Uh, and I guess mine was a prank of sorts too.