Minecraft wiki entry for Water, associated text: Each level of the Respiration enchantment adds 15 seconds to the breath meter and grants an x/(x+1) chance (where x is the Respiration level) of not taking damage after that time: 30 seconds and an average 1/second with Respiration I, 45 seconds and an average of 2⁄3 damage/second with Respiration II, and 60 seconds and an average of 1⁄2 damage/second with Respiration III.