The Player’s Handbook states the following when it comes to holding your breath:
“A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).Player’s Handbook, p183
When a creature runs out of breath or is choking, 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.”
This means that characters can hold their breath for 1 minutes + your constitution modifier with a minimum of 30 seconds. For example, a constitution 20 character (+5 constitution modifier) can hold their breath for 6 minutes. A character with 6 constitution (-2 modifier) can hold their breath for 30 seconds (as this is the minimum).
You then get a number of rounds equal to your constitution modifier where you are out of breath, but still conscious. If you run out of this time though, then you’re in big trouble. You’ll begin taking death saving rolls and can’t be stabilised until you get out of the water.
Bear in mind that to be rescued from this state, another character will need to swim you to the surface. Your swim speed is half movement speed (so typically 15ft per round).
A rescuer will also need to add encumbrance rules here which means if they’re carrying something weighing 5 times or more their strength score, then they are encumbered. This means even a 20 strength character will likely be encumbered carrying most characters. You may even be heavily encumbered if the character is 10 times or more your strength score. An encumbered character has a movement speed of 10ft less than normal and a heavily encumbered character has a movement speed of 20ft less than normal.
This means that you could quite easily end up with a movement speed of 5 or 10ft when trying to rescue a character from drowning.
The moral of the story, be really careful about falling unconscious when deep underwater, and make sure you have some contingency plans!
Fortunately, there are a handful of races that can hold their breath longer can breath water and/or have a swim speed equal to their movement speed. If you know you’ll be playing in an aquatic campaign, they might be worth considering as character options.
The races that can hold their breath longer or breath water are; Sea Elf, Air Genasi, Water Genasi, Autognome, Grung, Lizardfolk, Locathah, Plasmoid, Simic Hybrid, Tortle, Triton, Vedalken, Warforged
The races with a swim speed equal to their movement speed are; Sea Elf, Water Genasi, Giff, Lizardfolk, Locathah, Simic Hybrid, Triton
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