I once got "trapped" under one of those big diving rafts they put in lakes[1]. I swam down after one of those colored diving rings, and the wind blew the raft over me while I was under.

I was a pretty strong swimmer at that point, and more than a little lucky as well. I was able to collect my thoughts and get out.

I'm told I was only under there for about ten seconds, but my memory of it feels like about 2-3 minutes. It was easily the scariest thing that ever happened to me personally as a child.

[1]-Like this, but without the gap in the middle: http://www.aquacycleusa.com/aqua-cycle-swim-raft