When you are underwater and your oxygen bar is depleting restarting your world will cause the oxygen bar to reset to full when you are wearing a turtle shell helmet

Steps to Reproduce:
1. Load up any world in survival mode
2. Make sure you are wearing a turtle shell helmet
3. Go underwater and wait for the oxygen to bar to go down
4. Once the oxygen bar is low leave the world
5. Reopen the world and the oxygen bar will be reset