targets in water

How would you run a combat in which an attacker is above water or on land and the target is under 10 feet of water?

There are rules for underwater combat in the Player's Handbook (chapter 1 in the 5.5e version) though they're pretty bare-bones. Anything specific you're looking for?

In my head I was presuming the rules only covered fighting with both combatants underwater, not one in water and one not. I knew the forum dwellers would be able to point me in the right direction faster than looking around blindly in rules books.

It's pretty basic but I think it does cover this situation at least to some extent. Pretty much all it says is that things underwater have resistance to Fire damage, and lays out some limitations on making weapon attacks while underwater. 

The question would be whether someone outside the water attacking someone under the water would be subject to those same limitations. It sorta seems like they should be, though I don't see anything in the rules that actually says that.

The DMG also has rules on encounter distances based on water quality.

• Underwater Combat (PHB entry mentioned by wagnarokkr)
• Encounter Distance is not part of the DnD Beyond Free Rules.

What I would do for the actual encounter is:

• If a melee attacker or defender is fully or mostly underwater, the attack has disadvantage unless the weapon deals piercing damage or the attacker has a Swim Speed.
• Ranged attacks where the attack or defender is fully or mostly underwater automatically miss beyond the weapon's normal range.
  • This is simplified to avoid trying to figure out the difference between shooting a Short Bow into or out from water. I would expect out from to perform worse but shooting into more than shallow water may be hard to get the hang of as well.
• Walking in the water is difficult terrain unless the creature has a swim speed. Movement underwater is swimming.

In addition to what was said, if water is not clear but murky it could be Lightly Obscured or Heavily Obscured if more opaque.

I'm going to assume for this that we are focusing on ranged attacks/abilities. If that assumption is wrong, please let me know. The 2014 PHB (I don't have access to the 2024 version) states:

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.

I would just use these rules for both parties (the one in the water and the one outside of the water). This situation is unlikely to occur so often as to make up new rules for it. Ranged attacks will have disadvantage and only hit within the normal range. 

The creature in the water will have resistance to fire damage.

Beyond that, I don't think there's much to do. 

Unrelated to the question, I do find it very odd that a net or thrown weapon eliminates the disadvantage. I can see the crossbow, as I'm sure they are thinking it might behave similar to a speargun, but if you tried to "throw" a net underwater any distance it is going to get stopped by water resistance almost immediately. This would also be pretty true of thrown weapon I imagine. I think it would be very hard to throw something like a dagger with any amount of force behind it underwater. I know D&D isn't a physics simulator, but the fact that they specifically call out these weapons to bypass what seems like a reasonable limitation for underwater combat is a headscratcher for me. 

The 5.5e version is basically the same except that instead of a list of specific weapons, it just says that weapons that deal Piercing damage don't suffer the disadvantage. This is a lot easier to remember but it does have the same issues with verisimilitude you're mentioning.

Nets in 5e actually usually have disadvantage on all attacks anyway due to the way their range is defined. Nets are no longer weapons at all in 5.5e (and don't involve making an attack roll) so they got rid of that part.

This is definitely a house rule, but with one out of the water and the other in, I might give both sides disadvantage on attack rolls. The water makes things seem like they’re in a slightly different place, so it would throw off aim, and transitioning from air to water or vice versa will take a lot of the weapon’s momentum.  
As I said, house rule. But besides that, I’d just stick with the standard water rules. 

I would also give disadvantage to both sides with one in the water and one outside, assuming that the one in the water is below the surface. The piercing weapon can go through the water, but it's much harder to aim, and especially at any distance greater than 10-15'. And if they're more than 5' below the water the target is pretty obscured.

When you think about distance, angles are now a thing too. 5' below the water if the attacker is directly above you is only 5', but if the attacker is 30' away horizontally you're going through ~25' of water or you're counting on an arc above the water coming down in the right place.

Realistically there are a ton of problems with fighting underwater, but the result will tend to be complex and not much fun, so I'd go with something simple like:

• Attacker underwater: normal attack penalties for underwater combat, but the target does not get resistance to fire.
• Target underwater: normal attack penalties for underwater combat, but missile fire can go beyond short range as long as the excess range is out of water.

Quote from MaximusArael020 >>

I'm going to assume for this that we are focusing on ranged attacks/abilities. If that assumption is wrong, please let me know. The 2014 PHB (I don't have access to the 2024 version) states:

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.

 

[...]

 They are included in the Basic Rules (linked also by SmiteMakesRight_3_5):

Underwater Combat

A fight underwater follows these rules.

Impeded Weapons

When making a melee attack roll with a weapon underwater, a creature that lacks a Swim Speed has Disadvantage on the attack roll unless the weapon deals Piercing damage.

A ranged attack roll with a weapon underwater automatically misses a target beyond the weapon’s normal range, and the attack roll has Disadvantage against a target within normal range.

Fire Resistance

Anything underwater has Resistance to Fire damage (explained in “Damage and Healing”).