Underwater Combat

When adventurers pursue sahuagin back to their undersea homes, 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.

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.

If you would like underwater combat to be a bit more challenging, consider using the following additional rules:

A creature without a swimming speed that is wearing armor must succeed on a DC 10 Strength (Athletics) check at the beginning of its turn or sink straight down 10 feet and lose its movement for the turn. The DC for the check increases to 15 for creatures wearing medium or heavy armor, and creatures wearing heavy armor make this check with disadvantage.

A creature that takes damage while holding its breath must make a Constitution saving throw. The DC is equal to 10 or half the damage the creature takes, whichever is higher. If the creature fails, it loses its held breath and immediately begins to suffocate.

The surface of a body of water provides total cover to creatures on either side of it from creatures on the other side.