Turtle helmets doesn't give you water breathing while underwater, they only give water breathing when you are out of water
Comments 2
*Thank you for your report!
However, this issue has been closed as a Working as Intended.
If a player is wearing a turtle shell in their helmet slot while out of water or in a bubble column, they will receive Water Breathing for 10 seconds. This effect won't start counting down until the player goes underwater. This essentially gives the player 10 extra seconds of breath underwater.
https://minecraft.gamepedia.com/Turtle_Shell
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki
That is intended. The effect only lasts 10 seconds so it refreshes on land but it does not give you infinite water breathing. This is WAI.