Well I'm no expert but from what I've heard, one lungful can do you in, its a bit different when you are expecting it or even stumble into it because you brace yourself for it. A shove or a boat suddenly capsizing on you, one gasp "ooops thats not air" and you are unconscious/no longer breathing, someone would have to pull you out and revive you pretty fast.
The thing is though you'd have thought there would have been quite a few who did brace themselves in time, I'm surprised we haven't heard of anybody who has a tale to tell about it, maybe they just didn't take it that seriously and took it as a prank?