Minecraft wiki entry for Damage, associated text: A player who runs out of air underwater begins drowning, taking approximately 2 damage per second. Damage is taken when the air supply value reaches -20. Usually, the air supply value decreases each tick. It resets to 0 after damaging. Respiration gives a chance for air supply to not decrease itself per tick. Chance is x/(x + 1), where x is the level of enchantment.