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.
SWIMMING
Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re swimming. You ignore this extra cost if you have a swimming speed and use it to swim. At the DM’s option, gaining any distance in rough water might require a successful Strength (Athletics) check.
UNDERWATER COMBAT
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, and vulnerability to lightning and thunder.
CASTING SPELLS
Casting spells is possible underwater, though you will be limited to somatic and material component spells else you risk suffocating. If you choose to cast a spell with a verbal spell component, your character is no longer holding their breath and will begin suffocating as detailed below.
HOLDING BREATH AND SUFFOCATING
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.
For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.
TAKING DAMAGE
You make a Constitution saving throw with a DC equal to 10 or half the damage, whichever is higher, and a failure reduces the amount of remaining time that you can hold your breath by one minute.
If someone is concentrating on a spell while underwater, they must make two separate saving throws to maintain each instance of concentration.
Falling or Jumping into Water
When jumping into water or another liquid the damage is instead:
No damage for 20 feet of falling.
Half-damage next 20 feet.
Normal damage beyond that.
A creature that falls into water or another liquid can use its reaction to make a DC 15 Strength (Athletics) or Dexterity (Acrobatics) check to hit the surface head or feet first. On a successful check, any damage resulting from the fall is halved.
