The Underwater Combat rules on attacking state:
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).
When it says underwater combat, I assume this is true while both that attacker and target are underwater, but the rule doesn't seem to explicitly define whether this rule is triggered when:
- Both are underwater
- Attacker is underwater, target is on land
- Target is underwater, attacker is on land
Scenario 1 obviously follows the underwater combat rules, but how about 2 and 3? Do both of them follow the underwater combat rules?
Best Answer
The sentence immediately proceeding the quote in the question states (PHB p.198):
This does not say who is underwater, just that the combat is in some way underwater. The most straightforward and obvious way to interpret and play this would be to apply the rules as written when either party is underwater, unless the DM's common sense says otherwise (e.g. the target is only 1" under the surface of the water) or there is an explicit rule that states otherwise (e.g. the Wall of Water spell XGTE p.170).