Yep, drowning = death. Considering what's happening, it makes perfect sense.
Suppose we have a character of somewhat above average health, having a Constitution of 15. He's pulled under the surface suddenly. Normally he could hold his breath for 5 rounds, but since he was pulled under by surprise, that drops to 3 rounds. This is still impressive, as in 2nd Edition AD&D, a round is approximately 1 minute.
That character holds his breath for 3 minutes, THEN starts making Con checks. The first check is made against the base 15. Odd are, he'll make it. If he does, he's now been under for fully 4 minutes without a breath. Make another, and that's 5. In real life, not many humans can go 5 minutes under water unless they have an oxygen tank. Make another Con check, and that's 6. Most humans in real life can't even go a minute.
The next Con check needs to be under a 9, and is statistically unlikely to succeed, but it's still not too difficult. That's 7 rounds without a breath.
Odds are he won't make the next, and drowns.
7 rounds in AD&D is an eternity, and if a character can't surface in that time, he's probably already a goner anyway. I think if we were to go to hit point loss as the next step, it would become silly.
Actually, I think not tying it to hit point sis a wise move. It means a 1st level character and a 10th level character would last about the same time, as opposed to the level 10 guy living 10 times longer.
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