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Er, drowning pretty much kills you. With the six-second round, I've yet to have a pc run out of time to try something tricky before they run out of breath.
I have only seen a PC drown once since 3.0 is out and that was in the middle of the ocean where his companions where dealing with a couple of sharks as well (it did prompt my character to get a levitate spell in his spellbook). Most characters tend to be rescued easily within 10 to 18 rounds and hardly any PC will have a constitution below 10.