Suffocation and Drowning
A character in an airless environment (underwater, vacuum) can hold his or her breath for a number of rounds equal to his or her Constitution score. After this period of time, the character must make a Constitution check (DC 10) every round to continue holding his or her breath. Each round, the DC of the Constitution check increases by 1.
When the character fails one of these Constitution checks, he or she begins to suffocate or drown. In the next round, the character falls unconscious with 0 hit points. In the following round, the character drops to –1 hit points and is dying. In the third round after failing the check, the character dies of suffocation or drowning.