Everything You Need to Know About Underwater Combat

At some point, your players will likely want to try out the spell “Water Breathing” and will venture into the ocean, a river, or lake. Which means, you are going to have to know how to run combat underwater.

The idea of running combat underwater terrified me as a new DM! But it turns out the rule are intuitive, and it turned out to be one of the best adventures in the campaign! So don’t worry, its not as complicated as you might think!

How do you run underwater adventure in Dungeons & Dragons 5e? Here are some the rules to remember:

Unless a creature has a swim speed, its movement is halved
After each hour of swimming, a creature must succeed on a DC 10 Constitution Saving throw or gain 1 level of exhaustion
Melee weapon attacks have disadvantage (exceptions: dagger, javelin, shortsword, spear, or trident)
Ranged weapon attacks made outside of the base range automatically miss
Unarmed strikes have disadvantage
All creatures fully immersed in water have resistance to fire damage
a player can hold his/her breath for a number of minutes equal to 1+ their Constitution modifier (minimum of 30 seconds).
A player can last the number of rounds equal to his/her Constitution modifier (minimum of 1) when they run out of oxygen before they drown.
When a player begins to drown, their hit points fall to 0 at the start of their turn and they cannot receive the benefits of a healing spell or potion until they are able to breath again.

Movement Underwater

While in the water, movement speed is halved. Each foot of movement requires two feet of movement speed. For example, if a player normally has a movement speed of 30 feet, then he/she can only move 15 feet while in the water. This rule does not apply to players who have a swim speed.

It is also good to remember that while in water, players can move omnidirectionally as if they were flying. Players can move up, down, or diagonally allowing for some creative options during underwater combat!

However, this can be difficult to track on a regular battle map. I suggest picking up a couple of flight pads to track upward movement.

Vision Underwater

Not all water is clear and, depending on the depth, precious little sunlight may reach beneath the surface of the water. Use the following rules for vision when they apply:

Lightly Obscured– If the water is slightly murky or deep enough to be considered dim light, players have disadvantage on Wisdom (perception) checks which are sight-based.
Heavily obscured– If the water is very murky, sediment has been kicked up, or the depth is enough to heavily block sunlight, then players suffer the blinded condition. If players are blinded, then players automatically fail ability checks which rely on sight, attack rolls against the blinded creature have advantage, and the blinded creature’s attack rolls have disadvantage.

The range of vision for players in water also depends on the murkiness of the water. Use the following visual range for each type of water:

Clear Water, bright light: 60 ft.

Clear Water, dim light: 30 ft.

Murky water or no light: 10 ft.

Holding Breath Underwater

Unless your party has Water Breathing (either magically or naturally) then they are going to need to pay attention to their oxygen supply.

According to the Player’s Handbook, a player can hold his/her breath for a number of minutes equal to 1+ their Constitution modifier (minimum of 30 seconds). This element of underwater combat is often overlooked or neglected because it takes 10 rounds of combat for 1 minute to pass, meaning a player with a +3 Constitution modifier will have 4 minutes, or 40 rounds of combat before he/she begins to drown.

But, if you have ever been underwater then you know that movement and actions will decrease the amount of time for which you can hold your breath. Now imagine getting hit, stabbed, or grappled, how would those things affect how long you could hold your breath?

How you deal with this problem (if you see it as a problem) is entirely up to you. Because there isn’t a standard rule or roll for “losing your breath” here are some options you could use. Feel free to pick and choose, combining options to better fit your challenge level, experience, and game style:

When hit, players must roll a DC 12 Constitution saving throw. On a failure, players roll d100, the roll determines the percentage of oxygen they lose. For example, if a player has 1 minute’s worth of oxygen in their lungs and they roll 60, they lose 40 seconds (or 60% in rounds) of oxygen. Leaving them with only 4 rounds (40 seconds) before they begin to drown. (When using this rule, I always round down to the nearly tenth to make it easier.)
When a player is hit with a critical attack, they immediately lose all their oxygen and will begin to drown on their next turn.
When a player is surprised and does not have time to prepare to enter the water, then their oxygen supply is halved (again, to a minimum of 30 seconds).
Taking the Dash action automatically reduces a player’s oxygen supply by 2 rounds.

Drowning in Underwater Combat

When a player runs out of breath, they begin to drown. A player can last the number of rounds equal to his/her Constitution modifier (minimum of 1) when they run out of oxygen. At the beginning of his/her next turn, the player drops to 0 hit points and must make a death saving throw against a DC of 10. At the beginning of each of his/her turns after that point, the player must make a death save against a DC of 10. The player cannot regain hit points until he/she is stabilized and able to breathe.

Melee Weapons in Underwater Combat

Water is thicker than air, which means that weapons that depend on swinging motions to generate damage are at a disadvantage. Longswords, battle axes, staffs, glaves, and any other long weapon not designed to be thrown have disadvantage on attack rolls when underwater.

The only melee weapons which can be used effectively underwater are daggers, javelins, short swords, spears, and tridents. These roll attack as normal.

There is a way to avoid this penalty, however. Creatures with a “swim speed,” either granted to them naturally (such as a Triton) or magically do not have disadvantage on melee attacks rolls. If a creature has a swim speed, then their melee attack rolls are rolled as normal.

Unarmed strikes in Underwater Combat

Just like with melee weapons, unarmed strikes have disadvantage underwater. Because of the lack of force that can be generated, punches, kicks, and other unarmed melee attacks must roll with disadvantage.

This rule can make underwater combat especially challenging for a Monk. There is a monk monastic tradition called Way of the Four Elements includes Shape the Flowing River, which allows the monk to change a 30-foot cube of water to ice within a 120-feet. While this could be useful in underwater combat, there is still no monk ability within the Way of the Four Elements that would give a monk of that monastic tradition advantage on attack rolls underwater.

Ranged Weapons Attacks in Underwater Combat

Ranged weapons do not have disadvantage in underwater combat. Crossbows, nets, or a weapon that is thrown like a javelin (including darts) do NOT have disadvantage on attack rolls underwater. However, if ANY ranged weapon attempts to make an attack outside of its base range, the attack automatically misses.

For example, the base range of a short bow is 80 feet, even though it has a long-range of 320 feet, if a player attempts to make an attack with a short bow farther than 80 feet, the attack automatically misses.

This rule applies to all ranged weapons. For example, a hand crossbow does not have disadvantage on attack rolls underwater, but if a player attempts an attack outside of the hand crossbow’s base range (30 feet) then the attack automatically misses.

If you are thinking of having your players encounter enemies underwater, don’t let the rules of underwater combat scare you away!

Keep it simple and do what works best for your party. If you are comfortable running combat, add in some more realistic features. If this is your first time trying, don’t get so caught up in the rules that you forget to have fun.

Hopefully, this list will help you find what you need quickly to keep your game rolling.

Until next time,

may your game have advantage, my friends!

-Halfling Hannah