Underwater Adventuring

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 different and 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 (it will not reach that far or will not have enough energy to do any damage). 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, which includes a spear, trident or dart.

Creatures and objects that are fully immersed in water have resistance to fire damage.

Remember that creatures that do not have a swimming speed have reduced movement as swimming costs 2 feet for each foot moved, so swimming across a 5 foot square costs 10 feet of movement.

Finally, keep in mind that you will also want to incorporate the vertical elements of fighting underwater. It may be necessary for creatures or characters to swim up or down with their movement in addition to the normal horizontal options.