3e SRD:Swim Skill
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A successful Swim check allows the character to swim one-quarter of the character's speed as a move-equivalent action or one-half the character's speed as a full-round action. Roll once per round. If the character fails, the character makes no progress through the water. If the character fails by 5 or more, the character goes underwater and starts to drown.
If the character is underwater (whether drowning or swimming underwater intentionally), the character suffers a cumulative –1 penalty to the character's Swim check for each consecutive round the character has been underwater. The DC for the Swim check depends on the water:
Each hour that the character swims, make a Swim check against DC 20 or take 1d6 points of subdual damage from fatigue.
Instead of an armor check penalty, the character suffers a penalty of –1 for each 5 pounds of gear the character is carrying or wearing.