The 5e rules surrounding underwater combat are notoriously vague and leave a lot of room for interpretation. There are a few guidelines around breathing mechanics and movement and a couple limitations on attacks; but, for the most part, your party is at the mercy of your ruling.
For newer DMs, wielding that kind of power while establishing consistent and reasonable underwater combat rules can be tough. In this article, you’ll find the basic guidelines for underwater combat for Dungeons and Dragons 5th Edition and some discussion about the various interpretations DMs have used in their campaigns:
Attacks in Water
Almost any attack a player attempts underwater will be disadvantaged. It’s just not as easy to forcefully club an enemy over the head with a Warhammer or hit a target from a distance underwater. So, with few exceptions, any melee or range attack will have a disadvantage.
A disadvantage means the player should roll two d20 and take the lower of the two results. If they roll an 18 and 7, they have to take the 7.
Most melee weapons have a disadvantage because it’s hard to strike with any kind of force due to the resistance from the water. The exceptions to this rule are any weapons that work by piercing rather than by force of impact. Spears, tridents, daggers, javelins, and anything else that you could stab an enemy with doesn’t have a disadvantage.
Ranged weapons also have a disadvantage and are rendered completely useless outside of their normal range. Just imagine trying to shoot darts or use a slingshot underwater. It wouldn’t go very well.
There are exceptions here, as well, for piercing weapons. Players can throw spears and javelins or even shoot a crossbow without a disadvantage. However, they still can’t go beyond the weapon’s normal range.
Spells and Cantrips
There are no specific rules regarding spellcasting underwater which leaves this territory open to your own interpretation. For most spells and cantrips, it’s reasonable to assume that being underwater wouldn’t make too much of a difference.
There is some debate about verbal spells, though. Most DMs won’t allow it at all because it’s not really practical to speak underwater. Others allow verbal spells but dock minutes off the player’s breath as a penalty.
Basically, the player could pull off one verbal spell underwater in, say, a last-ditch effort to save the band of heroes from an encroaching frenzy of sharks, but they’ll need to immediately get their head above water afterward to avoid drowning.
Base your decision on how to handle spells and cantrips on your party. If you have a half elf sorcerer whose only really combat skills are spellcasting, you don’t want to limit their options too much. If nobody in your party depends primarily on magic, you could be stricter here to make the battle more challenging.
The Logistics of Underwater Combat
Water doesn’t just affect players’ attacks. It also poses a challenge to everything else. It’s hard to move. It’s hard to breathe. It’s hard to set enemies on fire.
Pro Tip: Don’t try to set anybody on fire when you’re surrounded by water.
For the purposes of calculating moving speed, water counts as difficult terrain. This is true whether the players are fully submerged, trudging across a river, or racing against time in a flooding dungeon to solve the puzzle that opens the door before they drown.
Because it’s difficult terrain, underwater movement should be calculated at half the player’s normal speed. So, a player with a walking speed of 30 feet per round would only swim 15 feet per round.
What About Characters with Swim Speed?
Some characters have a separate swim speed, usually because they’re an aquatic or amphibious race. In this case, skip the penalized walking speed and use their swim speed for their movement. Lizardfolk, for example, have a walking speed of 30 and a swimming speed of 30 so they would take no movement penalty in water.
Some DMs also waive ability checks for characters with swim speed. The logic here is that having a swim speed is a bit like having a proficiency bonus. Swimming isn’t just something the character can do if the situation calls for it. It’s something they’re actually good at so they’ll have more endurance and skill in the water.
By that same logic, you might also remove attack disadvantages for these characters. It would make sense that the Lizardfolk barbarian in your party — who probably spent their childhood playing in the dreary swamp where they grew up — would have learned a few things from getting into skirmishes with the other swamp dwellers. A human might find it difficult to wield a greatsword and tread water at the same time but the Lizardfolk will be a natural at it.
As a lung breather yourself (I assume), you probably know that breathing underwater is not quite the same as breathing air. This means that, unless the character has some special ability that allows underwater breathing, they will be holding their breath during this fight.
The question, then, is: how long should the character be able to hold their breath for?
For characters that do have a special breathing ability, their breath holding time will be listed as an ability. Lizardfolk, for example, can hold their breath for 15 minutes.
For most characters, though, you will calculate the time by adding one to their Constitution modifier. A character with a +2 constitution modifier could hold their breath for 3 minutes. Given that each round lasts 6 seconds, that’s 30 rounds.
If that character runs out of breath, they have 3 rounds (1+2 constitution modifier) to get above water. If they don’t, they start drowning and must cling desperately to life by making death saving throws until somebody brings them back on land.
By Rachael Green, contributing Dice Dungeons blogger.