Part of an ongoing series of 5e Rules notes.

5e has very much simplified (perhaps oversimplified, some argue) the issue of dealing with water as an obstacle, a location for fighting, and a danger. Always remember, D&D is not a physics simulator (or, as some have countered, it is a horrendously and hilariously bad physics simulator).

Note that there are a lot of exceptions below for creatures that have a Swimming speed in their stat block.

Movement in the Water

• Water is basically considered Difficult terrain if you don’t have a native Swimming speed. That is, each foot moved costs two feet of movement.
  • If the terrain within the swim is itself Difficult (e.g., a strong current, a kelp bed, etc.), this might increase to each foot moved costs three feet of movement.
  • You can, however, use any other movement speed (e.g., walking, flying) to swim with.
• If the water is “rough,” making any progress swimming might require a Strength (Athletics) check.
• There is (remarkably) no distinction in speed between swimming underwater or swimming on the surface.
• Don’t forget the Dash action, if you are doing nothing but movement.

Long-Distance Movement in the Water

If you have a Swimming speed, you can swim all day without penalty; use Forced March rules from the PHB.

Otherwise (per DMG 116), you need to roll a CON Save vs DC 10 for each hour swimming. Failure means +1 level of Exhaustion. Beyond that, there is a cap on 8 hours of swimming per day.

Deep Water

The pressures and temperatures of deep water take their toll. Per DMG 116, for creatures lacking a swimming speed:

• if swimming over 100 feet deep, makes every hour count as two for Exhaustion checks and limits.
• if swimming over 200 feet deep, makes every hour count as four for Exhaustion checks and limits.

Vision in the Water

• Clear water, bright light — 60 foot visibility to notice an encounter
• Clear water, dim light — 30 foot visibility to notice an encounter (Disadvantage to Perception).
• Murky water / no light — 10 foot visibility to notice an encounter (Disadvantage to Purcepti0n).

The above presume light sources or Darkvision.

Doing Stuff in the Water

By which we mean, of course, combat and magic.

Combat in the Water

When fighting underwater, again unless you have a native Swimming speed:

• Melee weapon attacks are at a Disadvantage (as you and/or your weapons are slowed by the drag of the water) …
• … except for a thrusting/piercing weapon like a dagger, javelin, shortsword, spear, or trident.

This melee weapon restriction Rule-As-Written would seem to apply to fighting while on the surface (swimming), or even while partially immersed (imagine fighting in waist-deep water); I’m not sure that last makes sense, and an appeal to the DM might be possible. (I can see any sort of melee weapon attack being at Disadvantage when standing in, but not under, water, as you are partially Restrained.)

• Ranged weapon attacks are possible underwater, but they are at a Disadvantage …
• … except when using a crossbow, net, or a weapon thrown like a javelin, spear, trident, or dart.
• They are an automatic miss if beyond normal range.

Magic in the Water

• You can cast spells underwater.
  • But if they have a verbal component, you have stopped holding your breath and have gone into step 2 of Drowning (below).
  • Material components may be difficult to manipulate while swimming or immersed in water.
  • Somatic components aren’t a problem, as only a single hand is needed for them.
• Spells that require a to-hit roll do so at a Disadvantage (if above water, because of aiming while trying to stay afloat; if underwater, because of the murk and visual distortion underwater, and drag on your body).

When you are fully immersed in water, you have Resistance to fire damage.

But aside from that, spells don’t do anything tricky. For example, lightning does not electrify the whole area. Remember (a) Bad Physics Simulator, and (b) it’s not actual lightning, it’s magic acting like lightning (handwaves).

Going “Prone” underwater

If something should knock you Prone while in the water, you are instead tumbling/floundering … but suffering the same status effects as being Prone (including slowed movement, Disadvantage to attack, Advantage to adjacent attackers) until you “stand up” / regain control with half your movement. (References 1, 2, 3).

Drowning

The rules here are essentially the same as suffocation rules:

1. You can hold your breath for (CON mod + 1) minutes (to a minimum of 30 seconds).
2. After that you survive (CON mod) rounds (minimum of 1) without needing more air.
3. After that, you are at 0 hp and are unconscious and dying. You can’t stabilize or heal until you can breathe — even if you make your three successful Death Saves, you only erase any unsuccessful ones and start the process over. You can be magically healed, but that can only get you back up to Step 2 unless you are out of the water by then.

Example: A creature with CON = 14 (CON mod = 2) can hold their breath for 3 minutes. After that, they start drowning/suffocating, and have 2 rounds to reach air before dropping to 0 hp.

Note that 3 minutes (or even 1 minute) is a ridiculous amount of time in the game (1 minute = 10 rounds of combat). People tend to be terrified of their character running out of air … the first time they get into underwater combat.  (It’s still a real threat, but not a close one.)

Workarounds

• Various races can either breathe underwater, or (such as Lizardfolk and Tortles) can hold their breath for longer, as defined in their stat blocks.
• Wild Shape and Polymorph can change folk into creatures that can swim or breathe in water.
• Anything that magically gives you a Swimming speed will be useful in the above.
• Water Breathing is a spell that literally lets you breathe underwater for 8 hours. It’s a 3rd level for Druids, Rangers, Sorcerers, and Wizards, can affect up to ten people, doesn’t require concentration, and can be done as a ritual. Alternately, Water Walk (same parameters) lets you walk on water and not worry about having to breathe it.

Wait, that’s it?

But aren’t there some classic tropes that these rule ignore?

Yes.

 

1. There’s no provision for heavy armor or a full backpack dragging you to the bottom, etc.; if you are strong enough to wear it, you are strong enough to swim in it (handwaves) … which is good, because it takes 5 minutes to remove heavy armor.
2. There are no provisions for using up breath faster if you are slowly flailing about with your greatsword instead sitting still and reserving oxygen.

Again, largely this is because D&D is a crappy physics (and biology) simulator, and intentionally so. The game design thought seems to be “Does this complication take away from the fun? Does it mean extra calculations, rolls, and otherwise bogging-down of the game? Then simplify or eliminate it.”

(Note to self: if I ever decide this is too simplified, this site has some interesting homebrewed additions.)