Hello everyone! In preparation for my players encounter with a black dragon who's lair is mostly a large underground lake (i.e. Forge of Fury), I did some research into the mechanics of swimming and underwater combat. First off, the RAW for both topics are left rather open and the community forums I have seen on the topic have various differing opinions on how to balance game mechanics and realism regarding the subject. Thus, I am sharing swimming and underwater combat mechanics I have implemented in my game for your own use.
Swimming and Underwater Combat:
Underwater combat follows the RAW in the core rulebooks, with the following clarifications and add-ons listed below.
In calm current or still water, players do not need to make an (Strength) Athletics to swim at half their movement speed, unless there is some additional obstacle or challenge being presented (i.e. something is trying to drag them underwater, they are fighting heavy current, or they are wearing encumbering armor
If a character is wearing encumbering armor (any heavy armor, or medium armor that imposes disadvantage on Stealth checks) then the player must succeed a (Strength) Athletics check to swim at half their movement speed regardless of the type of water they are in. The DC for this check is equal to the AC granted by the armor (e.g. half plate = DC 18, scale mail = DC 14). The player must make this check at the start of their turn regardless of whether they intend to use any movement on their turn.
If a player in encumbering armor fails their Strength (Athletics) check to swim, they expend all of their movement for the round treading water. If a player in encumbering armor fails their Strength (Athletics) check by 10 or more, they sink 5 feet and are subject to the rules for holding breath and drowning. If a player continues to fail their Strength (Athletics) check by 10 or more on subsequent rounds, they continue to sink 5 additional feet for every failure.
Vision and attacking submerged creatures from the surface:
If a creature is on the surface looking into water, their vison distances is halved and Perception checks are made at disadvantage.
Creatures outside of water attacking a creature in or underwater with ranged or thrown weapons are subject to the restrictions imposed for those weapon attacks in the RAW for underwater combat. Additionally, the creature underwater gains half cover (if only half submerged) or three quarters cover (if at least three quarters or fully submerged).
Holding breath underwater while in combat and/or casting spells.
The length of time a creature can hold their breath underwater remains the same under the RAW, with the following modifications listed below.
Creatures attempting to holding their breath while underwater, and are hit by an attack must make a constitution saving throw (DC = half the damage taken) or lose the air in their lungs. When a creature loses the air in their lungs, they have a number of rounds equal to their constitution modifier before they begin to drown.
Creatures that are holding their breath underwater may still cast spells with verbal components, but must succeed a DC 10 concentration check to successfully cast the spell. A creature that casts a verbal component spell underwater stops holding their breath, whether they succeed or fail casting the spell.
A creature that runs out of air may make a constitution saving throw DC 15 to push past their physical limits and avoid the effects drowning for one additional turn, and may make subsequent constitution saving throws each round with the DC increasing by 5 for every round thereafter.
- Critique my clarification of and expansion on holding breath, suffocating, and the relation between the two
- I looked through all my books for the highest athletics checks to see what “nearly impossible” refers to RAW
- Remember the non-attack combat actions
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