Minecraft wiki entry for Respiration, associated text: Respiration extends underwater breathing time by +15 seconds per enchantment level in addition to the default time of 15 seconds. It also grants a chance of not taking drowning damage each second by level / (level + 1). The maximum level that can be obtained without commands is level 3, resulting in a total time of 60 seconds underwater before drowning begins. Wearing a turtle shell extends this time by an additional 10 seconds.