Air Bladder (5e Equipment)
From D&D Wiki
This small, yet effective bag is made to hold tightly shut air inside of it. A medium size creature can hold its breath an extra 1 minute before it starts suffocating; however, a small size creature can instead holds its breath an extra 2 minutes before it starts suffocating.
For example, a medium size creature with a Constitution of 14 can hold its breath for 4 minutes. However, when it starts suffocating, it has only 2 rounds to reach air before it drops to 0 hit points.
Cost: 1 sp
Weight: 1/2 lb.