Drowning

A player can hold their breath a number of minutes equal to their Constitution modifier (minimum 30 seconds). If a player is holding their breath and takes damage, they make a con save with a DC equal to 10 or half the damage taken, whichever is higher. If they fail, they lose 1 minute of time for holding their breath.

If a player runs out of breath, they can continue without oxygen for a number of rounds equal to their Constitution modifier. When these rounds have passed, at the start of the player’s next turn, they immediately fall to 0 HP and take 1 level of exhaustion. Any player with 0 HP underwater (whether from losing breath or damage) makes death rolls on their turn as normal, but a player cannot regain hit points and cannot become stable while they remain underwater. If they make 3 death roll successes, they continue to make death rolls until dead or brought above water.