Underwater Combat

Swimming

Lakes and oceans simply require a swim speed or successful Strength (Athletics) checks to move through (DC 10 in calm water, DC 15 in rough water, DC 20 in stormy water). Characters need a way to breathe if they’re underwater; failing that, they risk drowning. When underwater, characters can move in any direction. Any character can wade in relatively calm water that isn’t over his head, no check required. Similarly, swimming in calm water only requires Strength (Athletics) skill checks with a DC of 10. Characters proficient in Athletics can swim easily in calm water without needing to make checks.

A creature without a swimming speed must succeed on a DC 10 Constitution saving throw for each hour it spends swimming or gain one level of exhaustion. If a creature has a swimming speed, it uses the standard travel pace rules in the Srd.

Stealth and Detection Underwater

How far you can see underwater depends on the water’s clarity. As a guideline, creatures can see 4d8 × 10 feet if the water is clear, and 1d8 × 10 feet if it’s murky. Moving water is always murky, unless it’s in a particularly large, slow-moving river.

Invisibility

An invisible creature displaces water and leaves a visible, body-shaped “bubble” where the water was displaced. Being invisible underwater gives a creature advantage on Dexterity (Stealth) checks, but on a failed check opponents can see the invisible creature’s location and size (though not its appearance or specific features). Creatures attacking an invisible creature have disadvantage on their attack rolls.

Falling and Diving into Water

If the water is at least 10 feet deep, the first 20 feet of falling do no damage. A fall of up to 30 feet deals 1d3 bludgeoning damage, while a fall of 40 feet or more deals 1d6 bludgeoning damage, plus 1d6 for every additional 10 feet fallen.

Characters who deliberately dive into water at least 10 feet deep (20 feet deep for fall over 60 feet) can reduce falling damage from a dive, treating the fall as 10 feet shorter with a successful DC 11 Strength (Athletics) or Dexterity (Acrobatics) check, plus an additional 10 feet shorter for every 1 point by which they exceed the DC.

Underwater Combat

When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.

A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Creatures and objects that are fully immersed in water have resistance to fire damage.

Drowning

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

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.

For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Deep Water

Very deep water is generally pitch black, requiring darkvision or other exceptional senses to navigate unless a light source is provided. In addition, in deep water the pressure of the water can impact traveling. A creature traveling at a depth greater than 100 feet but less than 200 feet treats every two hours of travel as if suffering from a forced march as detailed in the System Reference Document 5.1. A creature traveling at a depth greater than 200 feet treats every hour of travel as if suffering from a forced march.

The Bends

If a creature is more than 100 feet below the surface and ascends more than 100 feet in 1 minute, gas bubbles develop in its bloodstream from the rapid depressurization. The creature must make a Constitution saving throw at the end of that minute. The DC is 10 + 1 for every 10 feet it moved past 100 feet in 1 minute. On a failure, a creature suffers one level of exhaustion. Creatures naturally adapted to living in the ocean depths automatically succeed on this saving throw.

Cold Water

A creature more than 100 feet below the surface of the water must succeed on a DC 10 Constitution saving throw each minute or take 2 (1d4) cold damage. Creatures with resistance or immunity to cold damage or creatures naturally adapted to living in cold or deep water automatically succeed on this saving throw.

Moving Water

When water is moving swiftly, including rivers and rapids but also fast-moving currents and crashing surf your Strength Atheltics will be used to resist forced movement and related penalties.