Banknote's Underwater Combat Rules

In combat, holding your breath underwater is much more difficult. Creatures are not only swimming, but dodging, striking, taking damage, etc. As such, the rules for holding your breath in combat notably reduce the time creatures can hold their breath compared to what I call “Underwater Exploration Rules.” (For those rules, I use the guidelines in the DMG.) Unless directly contradicted, regular rules from the PHB and DMG apply as normal.

These rules apply to any creature that does not have the ability to breathe underwater.

Starting Breath

Just before a creature is submerged underwater, they take a Breath of air which provides them with enough air to last a number of rounds equal to 3 + their Con mod (minimum of 3). Each creature, at the end of their turn, reduces their remaining Breath by 1. If a creature is swimming within 5 ft. of the surface of the water, it can be assumed the creature is able to take a breath freely upon its turn. Any rounds spent this way do not count against the rounds holding their Breath, as they can reset their Breath to full.

Losing Breath

There are two primary ways creatures lose Breath while in combat: casting spells with verbal components, and taking damage. DMs are welcome to rule that other activities or hazards can threaten loss of breath as well.

Casting Spells

While it is slightly more difficult to cast spells with verbal components underwater, experienced spellcasters are able to do so reliably. However, doing so requires spending Breath. For each spell a creature casts that requires a verbal component, that creature must reduce their Breath by 2 to do so. If a creature only has 1 Breath of fewer remaining, it is unable to cast a spell that requires verbal components until it regains at least 2 Breath.

Taking Damage

When a creature takes damage while underwater, it is difficult not to expel some Breath in reaction, either in exertion or in grunts and gasps of pain. When a creature takes damage while holding its breath, it makes a Con save with a DC as if it were concentrating on a spell (DC 10 or half the damage taken, whichever is higher.) On a success, the creature loses no Breath. On a failure, the creature reduces their Breath by 1. Creatures concentrating on spells must also make Concentration saves separately, as normal.

Running out of Breath

If a creature begins its turn underwater while at 0 Breath, it is at risk of beginning to drown. The creature must make a DC 12 Con save. On a success, the creature endures the lack of Breath and may act normally. On future turns, the creature will continue to make Con saves (increasing the DC by 2 after each success) until they fail or regain Breath. On a failure, the creature begins to drown. They immediately fall unconscious, drop to 0 hit points, and are now dying. They must make Death Saving Throws as normal, and cannot stabilize until they are no longer drowning.

A creature that is unconscious and drowning may recieve healing as normal. However, they do not regain any Breath upon being healed. On future turns they must resume making Con saves as normal at whatever DC they had reached before they failed.

Sharing Breath

In dire situations, a creature may share its Breath with another creature. As an action, if a creature is able to breathe into an adjacent creature’s mouth, they may reduce their remaining rounds of Breath by any number to increase the other creature’s Breath by an equal number.

Grappling Underwater

Grappling underwater follows regular rules, however there is an additional option available to creatures. When taking the Attack action while grappling a creature, a creature may, in place of one of its attack, attempt to crush some of the Breath out of their target. Make an opposed Athletics check. If the attacker succeeds, the target creature loses 1 Breath.

Image Credit: Wizards of the Coast