When adventurers fight off sharks in an ancient shipwreck, or find themselves in a flooded dungeon room, they must fight in a challenging environment. Underwater, the following rules apply.
- Unless a creature has the ability to breathe in water, it must hold its breath to avoid suffocating.
- Unless a creature has a swimming speed, for each hour spent swimming it must succeed on an DC 10 Constitution saving throw or gain one level of Exhaustion. At the GM’s option, gaining any distance in rough water might require a successful Strength (Athletics) Skill Check.
- Swimming deep underwater imposes harsher tests of endurance: higher DCs on their Constitution saving throws, and additional Bludgeoning Damage when they fail. These tests affect even creatures with a swimming speed, though they have reduced DCs on their saving throws. See the Water Depth Effects chart below for a reference.
- While swimming in water colder than its body temperature, if a creature fails its Constitution saving throw, it is dealt additional Cold Damage equal to the Bludgeoning Damage taken.
- When making a Melee Attack with a weapon, a creature that doesn't have a Swimming Speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is water-native.
- A Ranged Attack with a weapon automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is water-native.
- All damage rolls on Attacks made with weapons by creatures that don't have a Swimming Speed are minimized unless the weapon wielded is water-native.
- Unless a creature has the ability to breathe in water, it cannot talk or cast a spell that has a verbal component.
- Creatures and objects that are fully immersed in water have Resistance to Fire Damage and are immune to the Ignited Condition.
Water Depth Effects
| Depth | Con Save DC | Creatures with swimming speed DC | Bludgeoning Damage on failed save |
|---|
| 100 ft | 10 | - | 1d6 |
| 1000 ft | 15 | 5 | 3d6 |
| 1 mile | 20 | 10 | 6d6 |
| 10 miles | 25 | 15 | 10d6 |
| 100 miles + | 30 | 20 | 15d6 |
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