Zardnaar
Legend
You can...leave...a diving bell, and go back into it. People can hold their breath a long time in dnd. It seems to assume really healthy lungs and practice in the skill of holding one's breath. (I'd have tied it to the Athletics skill, myself)
You don't need never-ending continuous breathing to explore underwater. You just need a place to breath within range, and a decent handle on how long you can hold your breath, and how fast you can swim.
People explored underwater before scuba gear.
I'll never understand folks who need to restrict mundanes on the most strict possible terms, even erring on the side of going beyond how restrictive real life is. Just let them be cool, guys. It's fine.
Not long term, I think Pearl divers in shallow areas could last around 8 minutes with a lifetime of training.