Swimming

The Swimming rules leave a bit to be desired (DMG116 and PHB175, 183, 198). Use the Rules as Written unless they conflict with the information here.

Swimming

Characters that have or acquire a Swimming speed do not use the rules related to movement. Characters with the ability to breath underwater do not use the rules related to holding breath.

Determine if a Swim check is required. Many times, it should not be. The characters are assumed to be accomplished swimmers capable of treading water, swimming under waves at the beach and swimming reasonable distances assuming conditions are reasonably good and they are suitably attired. In these cases, endurance matters more than skill, and this can be handled as outlined in the DMG if it becomes necessary. If the characters are not suitably attired or if conditions are not reasonably good, use the approach below:

  1. The DM sets the Base DC for the check. Calm water has a DC of 0.
  2. The DM adjusts the DC for each of the following, increasing by +5 each time one of the following applies:
* Strong current
* Great depth
* Very rough water / high waves
* Low visibility
* Debris in water
* Frigid or burning environments
* Substantial normal clothing worn (beyond swim-like attire)
* Light armor worn
* Medium Armor worn
* Heavy armor worn
* Carrying a heavy pack or other object

Each of these is cumulative, so a character wearing plate mail (heavy armor) and a thick fur coat jumping intto clear, placid water will have a DC modifier of +20 (substantial clothing, and 3 'steps' of armor worn). Doing the same in a raging ocean among the splinters of a shipwreck while trying to keep hold of the ship's strongbox invites a DC much higher than 30.

In these cases, characters make a Strength (Athletics) check. Failure indicates a lack of progress. A Failure by 5 or more indicates the character has been drawn underwater 5ft by each increment of 5 the character failed by. Example: DC 20, character rolls a 9. The character fails to make progress and sinks 10ft underwater. Subsequent failures will cause the character to sink further.

Success on a Strength (Athletics) check allows swimming movement per the PHB: all movement is considered through difficult terrain. Characters may take the Dash action while swimming, but this must be declared prior to the roll being made. A second roll is not required; the entire Swim attempt is handled via one roll.

Note that the character's strength and encumbrance rating do not play a role here. This is because of the Archimedes Principle and the buoyant force. Having a density (one's weight per unit volume) smaller than or very near the density of water is what makes swimming possible. High weight over a small volume is what causes one to sink, independent of how fit or strong the character is. Fitness and strength do help one recover from a "failure to be buoyant" however.

Holding Breath

Characters can hold their breath per the DMG (1+ CON modifier in minutes, minimum 30 sec). However, this assumes placid, lotus-like activity - holding one's breath while remaining calm and moving as little as possible. Substantial movement such as swimming in attire other than something light and thin, fighting, carrying a heavy load, walking along the bottom or being in especially unfavorable conditions reduces this time by half (or more).

Endurance

The Endurance rules for swimming in the DMG call for checks every hour or more often under certain circumstances. These rules are fine if the swimmer is properly attired. However, characters bearing heavy loads because of clothing, armor, burdens or all three will become exhausted much more readily than described. To model this, any time a character fails a Swim check, they must also make an immediate Endurance check as described in the DMG. Failure adds a level of Exhaustion that can be only removed by the character resting on the surface in some manner, such as escaping the water altogether or clinging to a piece of flotsam. Treading water while burdened is not sufficiently restful to remove this level of exhaustion.

Walking Along the Bottom

Characters drawn to the bottom of a body of water may walk upon the bottom. All movement is treated as difficult terrain, or especially difficult terrain if the bottom is itself difficult to navigate. Walking along the bottom of a body of water may stir up blinding silt, making navigation impossible.