Reading all this made me realize: Convertible cars/trucks don't have a chance underwater, do they? The passengers would find the top flattened down on them in an instant.
OK, I'm scratching my head on this. Really, if the roof of a convertible "collapsed," wouldn't that be just some cloth bumping your head? Just cut it open (yeah, it's tough cloth, but softer than steel).
And before you worry about tons of water on you, it's not like anyone that hasn't dived a few feet underwater didn't have water on them. All that water isn't focusing it's weight on a specific spot - or else scuba divers wouldn't get very far.
Someone correct me if I'm wrong.