Swimming: distinguish movement cost from rough-water checks

SRD 5.2.1 · Movement

At the table

1. Without using a Swim Speed, each foot swum costs two feet of movement.
2. Swimming in Difficult Terrain costs three feet per foot under the entry’s two-extra-feet rule.
3. A Swim Speed removes this extra swimming cost; rough water may require DC 15 Strength (Athletics) at the GM’s option.

Example decision

A character without a Swim Speed has 30 feet of movement and swims in ordinary water with no Difficult Terrain. How far can that movement take them?

15 feet.

Swimming adds one extra foot of movement per foot traveled, for a total cost of two. Thirty feet pays for fifteen feet of swimming. A rough-water check addresses whether the attempted swim succeeds under those conditions; passing such a check does not itself remove the movement surcharge.

Watch for this mistake

Do not require every calm-water swim to use the optional rough-water check. Read the actual conditions and any creature-specific movement feature before setting a roll.

Ask the table

Is a Swim Speed being used, what does each foot cost, and is the water rough?

Source rule

While you’re swimming, each foot of movement costs 1 extra foot (2 extra feet in Difficult Terrain). You ignore this extra cost if you have a Swim Speed and use it to swim. At the GM’s option, moving any distance in rough water might require a successful DC 15 Strength (Athletics) check.

Source: SRD 5.2.1, 189. Online guide.