Suffocating

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30s).

When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round).

At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.