Underwater Combat

Unless aided by magic, a character can’t swim for a full 8 hours per day. After each hour of swimming a character must succeed on a DC 10 Constitution saving throw or gain one level of exhaustion.
A creature that has a swimming speed-including a character with a ring of swimming or similar magic can swim all day without penalty and uses the normal forced march rules in the Player’s Handbook.
Swimming through deep water is similar to traveling at high altitudes, because of the water’s pressure and cold temperature. For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.

When making a melee weapon attack, 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 a dagger, javelin, shortsword, spear, or trident.
A ranged weapon attack 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 a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
Creatures and objects that are fully immersed in water have resistance to fire damage.

Visibility underwater depends on water clarity and the available light. Unless the characters have light sources, use the Underwater Encounter Distance table to determine the distance at which characters underwater become aware of a possible encounter.

ConditionsEncounter Distance
Clear water, bright light60 ft.
Clear water, dim light30 ft.
Murky water or no light10 ft.

Underwater Combat

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