Deep underwater, a sunken ship marks the entrance to a deep underwater cave. But to pass through the ship, the party will have to deal with its sub-nautical guardians…
This week, we’ll learn about how to run underwater combat in DnD 5e with a special ruleset designed to highlight the differences between fighting underwater and above it. We’ll want to make the two feel different without slowing down underwater combat too much with extra rules.
Below the waves, where sunlight dapples on the water’s surface, the water gets cold and dark quickly. At the edge of the deep sunlight, where that blue light begins to fade, an old wooden ship is wedged between the rocks on the ocean floor.
The ship’s hull has been scratched by the claws and teeth of some huge beast in times long ago, and it is covered in silt and bottom-feeders as it slowly decomposes at the bottom of the ocean. But the old wreck contains a secret: inside its dark interior is the entrance to an underwater cave. If the players wish to access the cave, they’ll need to pass through the old wreck. And they won’t be alone…
How to Run Underwater Combat in DnD 5e
This encounter is an excuse to talk about how to run underwater combat in DnD 5e. We’ve placed it in the 2nd tier of play because that’s where it’s the most fun, in our experience: the characters have the tools to explore underwater encounters, but the challenges of underwater combat are still serious enough that they need to be planned around.
The characters will need to plan around holding their breath and drowning in order to fight underwater. Assume each PC can hold their breath for a number of minutes equal to their CON modifier (minimum 1). This will allow them to swim around and at least check out the situation underwater. Then, when they run out of breath, according to Chapter 8 of the Player’s Handbook:
Want a proper pirate encounter? Check out Making Waves.
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.
Read more about drowning
Water breathing is reasonably accessible via magic items and other magical effects at this tier of play. Hopefully your players have a plan for dealing with that.
Underwater combat affects the 5e combat rules in other ways. Here’s a recommended list of rules for running underwater combat. We use all of them, but you should just use the ones you think will be fun for your game (in particular, the movement ones are most fun when you use a battlemap).
- Movement speed is halved (water is difficult terrain).
- Ranged projectile weapons are ineffective underwater.
- Melee weapons that deal slashing or blunt damage take a -4 penalty to hit underwater.
- Spells that your group reasonably thinks could be cast underwater can be, as long as the caster describes how they adapt or change the spell to be cast underwater.
- Due to the three-dimensional possibilities of movement, creatures can pass through each other’s spaces unhindered, though doing so through an enemy’s space provokes an attack of opportunity.
These simple changes make combat feel very different, as the party will have to rely on a different set of weapons and spells than they usually do.
We’ll select our enemies based on the location and the character’s goals to better suit these changes to the combat rules for underwater play.
Goals
Using These Underwater Combat for DnD 5e Encounters
Think about what your players want and why they’re here, far under the waves. We’ll use the party’s motivations to design the enemies and and specifics of how to run underwater combat in DnD 5e. Why are your players looking under the waves? What brings them down to this old wreck? We’ll adapt our encounters based on what they want.
- If your players are just passing through the ship in search of something else, or they’re hunting a large underwater monster, we’ll use monsters that can pin them down and prevent them from moving on. Run Encounter A.
- If your players are swimming up through the water and trying to escape to the surface, or fleeing something else chasing them, we’ll use monsters that can set a trap and ambush them as they flee. Run Encounter B.
- If your players are diving for buried pirate treasure, we’ll need powerful underwater pirate-themed guardians, cursed forever to guard the plunder the party is seeking. Run Encounter C.
Encounter A
The sunken ship’s hull is stuffed full of giant sharks and deep-water elementals. If the party wants to access the cave, they’ll have to fight their way through.
How to Set Up this Underwater Combat Encounter
In this encounter, the party wants to access the cave and must run a gauntlet of giant sharks and water elementals to reach it. The party doesn’t need to kill everything here to get through, and may even be able to sneak past most of the guardians.
As the players approach the wreck, describe the bones that litter the sandy ocean floor around the wreck, and describe a few sleek shapes that swim endlessly in the dark waters inside the wreck. The players will need to swim quickly and carefully to make it through these waters unharmed.
If they’re holding their breath to do this, remind them that holding their breath while taking damage requires a Concentration check, and they might lose their breath if they fail! If this seems too punishing, consider removing a minute from their breath-holding on a failed save instead.
How to Run this Underwater Combat Encounter
The beasts that dwell down here in the wreck have formed a symbiotic relationship: the water elementals ensnare anything big that passes through, and then the giant sharks eat it, leaving scraps to be easily absorbed and dissolved by the elementals.
That means the water elementals will focus on locking down the PCs’ movement as they try to move through the wreck, and the sharks will focus their damage in a feeding frenzy on anyone caught by the elementals.
In encounters that only feature water elementals, have the water elementals focus on locking down the movement of the players, one by one. They want to prevent any party members from leaving their territory!
Underwater Encounter Generator
| Avg. Level | Num. Players | Water Elementals | Giant Sharks |
| 5 | 3 | 1 | 0 |
| 5 | 4 | 1 | 0 |
| 5 | 5 | 1 | 1 |
| 5 | 6 | 1 | 1 |
| 6 | 3 | 1 | 1 |
| 6 | 4 | 1 | 1 |
| 6 | 5 | 1 | 2 |
| 6 | 6 | 1 | 2 |
| 7 | 3 | 1 | 2 |
| 7 | 4 | 2 | 2 |
| 7 | 5 | 2 | 2 |
| 7 | 6 | 2 | 3 |
| 8 | 3 | 2 | 2 |
| 8 | 4 | 2 | 3 |
| 8 | 5 | 2 | 3 |
| 8 | 6 | 3 | 3 |
How to Resolve this Underwater Combat Encounter
If the players make it through, they’ve accessed the underwater cave and can continue their adventure! Reward them by allowing them to control movement through the area.
An easy way to do this is to have them discover a pulley that moves a grate on the ship’s old deck, controlling access to the cave entrance. Having the players be able to control access to the cave, and even block it off, if necessary, is a good way to give them more control over their movement down here.
If it’s an appropriate time for a treasure hoard, you might also have them find some old abandoned trade goods in the ship’s hold, such as crates of rare ores or gems that haven’t rotted away. These crates, if they can be transported, are worth 2d6 x 1000 gp to the right buyer.
Encounter B
As the group makes their panicked way to the surface, gasping for breath, they’re ambushed by a group of sahuagin fanatics!
Underwater Encounter Generator
| Avg. Level | Num. Players | Sahuagin Priestesses | Sahuagin Barons |
| 5 | 3 | 0 | 1 |
| 5 | 4 | 1 | 1 |
| 5 | 5 | 1 | 1 |
| 5 | 6 | 2 | 1 |
| 6 | 3 | 1 | 1 |
| 6 | 4 | 2 | 1 |
| 6 | 5 | 2 | 2 |
| 6 | 6 | 3 | 2 |
| 7 | 3 | 1 | 2 |
| 7 | 4 | 2 | 2 |
| 7 | 5 | 3 | 2 |
| 7 | 6 | 3 | 3 |
| 8 | 3 | 2 | 2 |
| 8 | 4 | 3 | 2 |
| 8 | 5 | 3 | 3 |
| 8 | 6 | 4 | 3 |
Setting up this Underwater Combat Encounter
In this encounter, the party is fleeing from the underwater cave below and has to make it up through the wreck to the surface. Unfortunately for them, the wreck is crawling with Sahuagin Priestesses and Sahuagin Barons who like to use land-dwellers as sacrifices to their dark gods. The party isn’t required to exterminate every foe to survive – agility and cunning may serve them better than brute force.
As the players approach the wreck, emphasize the eerie, scattered remnants of splintered wood and coral floating in the brightening water. Up above, the sinister silhouette of lurking sahuagin keep an ominous vigil. The players will need to swim swiftly and stealthily to evade these dangerous waters unscathed. We’ve prepared for this like a combat encounter, but it doesn’t need to be—if the players are quick and quiet, perhaps they can escape without being noticed.
If the sahuagin know they’re coming, though, that’s a different story. If the player made a big noise in a previous encounter, or a message reached these sahuagin somehow, they’re prepared. Give the sahuagin advantage on their Wisdom (Perception) checks to notice the players.
As the players enter the wreck, describe the coral-scratched carvings covering every surface and the shrine the sahuagin have made, littered with bones. The atmosphere should be savage and menacing.
Remind the players that if they’re holding their breath, taking damage necessitates a Concentration check. A failure could mean they lose their breath! If this mechanic seems overly punishing, consider deducting a minute from their breath-holding time on a failed save instead.
How to Run this Underwater Combat Encounter
The Sahuagin inhabiting this wreck have established a simple but effective strategy: the Sahuagin Priestesses use hold person to capture the weakest characters they see, setting them up for the Sahuagin Barons to defeat, then capture. The captured prey is then destined for a grim sacrifice to their savage gods.
This means the Sahuagin Priestesses will concentrate on landing those crucial hold person spells as they try to navigate through the wreck, while the Sahuagin Barons will focus on defeating, then capturing (as in, carrying off to another location) those ensnared by the preistesses.
In encounters featuring only Sahuagin Priestesses, these creatures will aim to hinder the movement of the players, one by one. Their goal is to carry away at least one suitable sacrifice, but they’ll take everyone they can reasonably get! Once half the sahuagin have fallen, the others will flee—it’s not worth it for a few sacrifices.
Resolving this Underwater Combat Encounter
If the players successfully escape, they’ve reached the surface and can continue their adventure! Their lives should be treasure enough for overcoming the ambush.
But if they manage to defeat all the sahuagin and poke around the wreck a bit, they might find a few valuables. Sahuagin don’t make a habit of gathering gold, but it’s possible they threw some crates of trade goods to the side when clearing room for their dark shrine.
If it’s a fitting time for a treasure hoard, consider letting the party discover some long-forgotten bounty in the ship’s hold – crates of precious ores or gems that have withstood the ravages of time. These crates, if successfully transported, could be sold for 2d6 x 1000 gp to an interested buyer.
Think about where else the sahuagin might be active. Is there a sunken temple nearby? Maybe there are more treasures to plunder. Consider adapting The Greatest of Us to this underwater setting and using something like the Breath of the Sea (in Encounter C, below) as the great prize.
Encounter C
The wreck of the old captain’s ship is only a few fathoms down, they say… but the gold is cursed and guarded by the wretched crew who plundered it in life!
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