Expanded Underwater Combat

If you would like an underwater fight to be a bit more harrowing than the standard guidelines on underwater combat, consider using the following additional rules:

A creature without a swimming speed that is wearing armor must succeed on a DC 10 Strength (Athletics) check at the beginning of its turn or sink straight down 10 feet and lose its movement for the turn. The DC for the check increases to 15 for creatures wearing medium or heavy armor, and creatures wearing heavy armor make this check with disadvantage.

A creature that takes damage while holding its breath must make a Constitution saving throw. The DC is equal to 10 or half the damage the creature takes, whichever is higher. If the creature fails, it loses its held breath and immediately begins to suffocate.

The surface of a body of water provides total cover to creatures on either side of it from creatures on the other side.