DnD 5e - Underwater Combat
Not every fight will take place on dry land. Sometimes you'll find yourself in the unfortunate poisition of fighting underwater. Maybe you pursued an aquatic foe like a tribe of sahuagin back to their sunken home. Maybe you were dragged off of a ship by an attacking karken and dragged underwater. Maybe you're in a rapidly flooding dungeon room. Or maybe you just picked a really bad time to take a bath. Either way, you're underwater and you're fighting for your life.
Fighting underwater adds two new problems on top of the already life-or-death nature of combat. First: you're probably holding your breath to avoid suffocation (see Environment, earlier in this guide). Second, many attacks and spells may not be as effective underwater, so your favorite weapon may suddenly be ineffective.
If you learn nothing else from this page, remember this: always carry a dagger. It's a useful all-purpose tool, it works while fighting underwater, and they're an inexpensive backup weapon even when you're not fighting disgruntled fish. If you can handle the extra weight, a spear and some darts are good idea too unless you can cast spells to attack at range.
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 (meaning that attacks at long range automatically miss rather than simply suffering Disadvantage). 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.