The Minecraft Wiki is no longer considered as official by Microsoft and therefore several changes are required to be made, including to the wiki's logo. Please read this announcement for more information.
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.