I was very close to drown when diving, and someone planted a 6 cm high, and 2*1 meter wide foam block[0] on top of where I had planned to surface, and I was caught underneath without much air.
I've never been close to drowning from something like that, but I've definitely felt that panic set in when you start swimming for the surface and then bump into a big float that wasn't there when you went down
[0] http://shopdolphin.dk/139h/images/35111.jpg