Sometimes, as a DM, you just want to mix it up and throw your players out of their comfort zones.

A great way to do that is with underwater combat.

There are a ton of fantastic options for adventures that take players beneath the waves. Shipwrecks, sunken temples, and coral labyrinths can be exciting and memorable experiences!

Sure, there are different rules to be aware of when you’re running underwater combat, but don’t worry about it!

So if you’re looking to put your players up against the denizens of the deep, let’s go ahead and dive in!

This article will give you everything that you need to know about how to run combat underwater in D&D 5e!

Movement

Let’s start with the most important thing: underwater movement.

We talked about swim speeds a bit in a previous article on movement in D&D 5e, but we’ll review it here! (Though you should still check out the article. I think it’s one of the most important ones I’ve written so far.)

Unless a creature or character has a swim speed, underwater is considered to be difficult terrain. This means that a creature or character moves at half of their normal speed. When navigating the combat, this is vital to know!

If a character does have a swim speed, they can move normally up to that speed.

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.

Exhaustion and Drowning

Just a couple more important considerations to discuss before we get to talking about weapons and spells.

Exhaustion and Drowning both become incredibly important and dangerous factors to be aware of when your party is underwater. In a perfect storm of a situation, these can directly lead to a TPK. Not to mention, your players will likely never be able to make a character who isn’t aquaphobic again.

Exhaustion When Swimming

The Dungeon Master’s Guide includes a small section for helping DMs plan underwater encounters. Planning encounters is a bit outside of the scope of this article, but let me know in the comments if you’d like to see a more in-depth guide about that.

The section does, however, give some more information about swimming that is not presented in the Player’s Handbook.

According to the DMG, players who aren’t aided by magic must make a DC 10 Constitution saving throw or gain one level of exhaustion for each hour that they are swimming.

Furthermore, swimming at a depth of greater than 100ft makes every hour count as 2 hours for purposes of determining exhaustion. Want to go greater than 200ft deep? Each hour counts as four!

I get the purpose of these rules as they are meant to reflect the effects of the increased pressure on characters’ bodies. But, this adds a ton of extra stuff to have to keep up with.

Honestly, it’s better for everyone if you just make sure to give the players some kind of magic item that negates the need for these rules. Otherwise, one failed Constitution save and they’re all going to go right back to where they came from.

Drowning in D&D 5e

A character can hold their breath for a number of minutes equal to 1 + their Constitution modifier. If they have a Constitution score of less than 10 for some reason, they can hold their breath for 30 seconds. At that point, the character falls unconscious.

Once unconscious, the character can survive for a number of rounds equal to their Constitution modifier (minimum of 1 round.) At that point, the character drops to zero hit points and must begin making death saving throws.

When drowning, a character cannot regain hit points or be stabilized until they are once again able to breathe.

If an adventure is taking the party underwater, they should absolutely have some method of gaining the ability to breathe. Whether this is with the Water Breathing spell or some other effect, it’s a must-have.

Of course, these rules might still come into play with a nasty use of Dispel Magic while the party are underwater… *evil laughter*

Weapons and Underwater Combat

Not all weapons are exactly ideal for fighting underwater in D&D 5e. Realistically, there’s actually a small number of weapons that aren’t difficult to use!

So let’s go over melee and ranged weapons and how they work (or don’t) underwater.

Melee Weapons

When underwater, weapon attacks are made at disadvantage.

There are a few exceptions, however. Some (though not all) piercing weapons work well underwater as one would expect due to having less resistance from the water hindering the attack.

To roll attacks normally, characters have to attack with a dagger, javelin, shortsword, spear, or trident.

Though there is one other way to get over the disadvantage from attacking underwater with a different weapon…

Keep in mind that having a swim speed overcomes this. Not only can you move more freely underwater, but you can attack normally with any weapon if you have a swim speed.

Ranged Weapons

If you’ve ever tried to shoot an arrow or throw an axe underwater, you probably learned two things:

  1. It’s horribly ineffective.
  2. It’s a quick way to get banned from using the pool.

Much like melee weapons, there are only a few ranged weapons that won’t inflict disadvantage on your attack rolls underwater.

Crossbows, nets, javelins, spears, tridents, and darts are able to be used normally underwater.

For all ranged weapons, even those that aren’t subject to disadvantage, they must be used within the weapon’s normal range. Going beyond that, the attack automatically misses.

So a spear, for example, has a normal range of 20 feet with a maximum range of 60ft. When throwing a spear underwater, you can only throw it up to 20 feet or the attack will automatically miss.

For more information on how ranged combat works in D&D, check out this other article where we fully explore the mechanics of using ranged weapons.

Spells

And finally we come to spells.

The vast majority of spells will work fine underwater. The DM should read the spell’s description if there’s any question and make a ruling that makes sense.

Remember: you want to keep the game moving and not turning into a book-thumping philosophical debate.

But there are two types of damage that very commonly cause confusion, so we’ll cover those!

Do Fire Spells Work Underwater?

Using the Rules As Written, the only difference in the effect of fire spells when cast underwater has more to do with the targets.

All creatures have resistance to fire damage when submerged underwater.

This means that spells like Fireball or Fire Bolt will only be doing half damage at best.  

The general reasoning behind this is that the spell itself is magical fire which allows it to still be cast. However, it can’t actually ignite anything because it’s still underwater.

I’ve seen multi-page debates about the physics behind this.

My thoughts: fewer things are less fun to me than physics debates. It’s a fantasy game. Make a ruling, be consistent with it, suspend disbelief, and play the game.

What Happens To Lightning Spells Underwater?

Much like with fire spells, nothing special happens when using lightning spells underwater if you are using the Rules As Written.

As a DM, you might throw in some extra flair when describing the effect of the spell. However, there isn’t a clearly written or defined benefit to using lightning underwater.

This once again follows the logic of “it’s magic and will therefore react differently” and relies on suspension of disbelief.

My Personal Homebrew

I wanted to make a clear distinction between my personal homebrew rules and the Rules As Written. These may not work for everyone, but these are my rulings at my table.

Fire spells can still be used, but have disadvantages. In addition to creatures resisting the damage for being underwater, the range and AoE of fire spells are both halved. In the case of AoE spells like Fireball, creatures have advantage on their saving throw.

Lightning spells do receive some bonuses. Creatures have disadvantage when attempting to make a save against an AoE lightning spell. The range and AoE of lightning spells are doubled underwater.

It’s quick and simple. The party’s Storm Sorcerer was happy and, ultimately, creating situations where players can feel awesome (and maybe a little overpowered) is pretty fun. He had to mindful of not also zapping his friends, but we were all happy with this ruling.

Conclusion – How to Run Combat Underwater in D&D 5e

Underwater combat in D&D 5e is certainly different, but it’s still relatively straight forward.

Even despite the restrictions that it puts on some characters, it can be very useful for shaking up your game.

After some period of time, characters will commonly find themselves relying on the same tactics more or less in every combat. Mixing it up by taking them underwater takes them out of this comfort zone and opens up a new exploration of their character’s abilities.

Plus, now having to do combat in three dimensions opens up an astounding amount of positioning tactics!

Still got questions about running combat underwater in D&D 5e? Let’s chat in the comments!