Armor Penalty

Using this skill, a land-based creature can swim, dive, navigate underwater obstacles, and so on.

Check: A successful Swim check allows you to swim one-quarter your speed as a move action or half your speed as a full-round action. Roll once per round. If you fail, you make no progress through the water. If you fail by 5 or more, you go underwater.

If you are underwater (from failing a swim check or because you are swimming underwater intentionally), you must hold your breath. You can hold your breath for a number of rounds equal to your Constitution score, but only if you do nothing but take move actions or free actions. If you take an attack action or a full-round action (such as making an attack), the amount of breath you have remaining is reduced by 1 round. (Effectively, a character in combat can hold his breath only half as long as normal.) After that period of time, you must make a Constitution check (DC 10) every round to continue holding your breath. Each round, the DC of the check increases by 1. If you fail the check, you begin to drown.

The DC for the Swim check depends on the water:

Water DC Calm water 10 Rough water 15 Stormy water 20