Well I didn't get your answers until after the session, so HRs it was. They killed the creature with the aura of drowning, and baring any underwater dispel magic it is unlikely to happen again. (although a dispel will be an option in an upcoming battle.)
after making what I could out of this artical
http://www.emedicine.com/ped/topic2570.htm
I have pulled the following facts:
"Death by drowning occurs within 24 hrs of immersion. "
"Success or failure of initial basic life support provided at the scene of the accident is the most important determinant of outcome."
"Patients with completely normal findings on examination and trivial history may be discharged after a 6-hour observational period."
"The most significant impact on mortality occurs before the patient arrives at the hospital."
"Many pateints suffer secondary complications and die from pulmanary problems, hypothermia or pneumonia."
So my educated House Rule is if the drowning person is saved at 0 hp, then its non-leathal damage. If they reach -1 hp it is leathal. A successful healing check or healing spell provided in the round the patient stops drowning reduces the effects by 1 step.
I realize this is overly complicated, and converting actual medical effects to D&D damage is inheritantly flawed.
Any one else have an option, rule or reality based?