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Aquatic Rules / Underwater Movement

Swimming Without a Swim Speed

Source Starfinder #36: Professional Courtesy pg. 47
A creature that takes a move action to move its speed while underwater must choose a direction to swim (including up or down) and attempt an Athletics check to swim. The DC of this check is 10 in calm water but increases in rough conditions (DC 15), stormy conditions (DC 20), and maelstrom conditions (DC 30). Strong currents, debris, and other environmental conditions can increase this DC further. A creature can’t take 10 to swim in combat or in stormy or maelstrom conditions, even if not distracted or in immediate danger.

On a successful check to swim, the creature moves half its land speed in a straight line in its chosen direction. If the creature wants to change direction (including ascending or descending) while swimming, it costs an additional 5 feet of movement to turn 45 degrees. See the Swimming Example below for an example of a creature swimming and changing direction.

If a creature fails a check to swim by 4 or less, it makes no progress. If it fails by 5 or more, it sinks, moving 5 feet directly downward.

Every hour while swimming, a creature without a swim speed must succeed at a DC 20 Athletics check or take 1d6 nonlethal damage from fatigue.