Could you add a mod to the helmet that grants underwater breathing? Since I'm at the endgame stage and want to build things underwater—like a large 100x100 block structure—180 seconds just isn't enough time.
This belongs belong to the Pimp Dreams section since you are wanting a feature to be added to the game. Otherwise, check out Game Modifications. They might be able to suggest a mod that fits your request.
There is also Nexus and 7DaystoDieMods, two popular websites for modding the game