You know how to swim and can do so even in stormy water.
Make a Swim check once per round while you are in the water. Success means you may swim at up to half your speed (as a full-round action) or at a quarter of your speed (as a move action). If you fail by 4 or less, you make no progress. If you fail by 5 or more, you go underwater.
If you are underwater, either because you failed 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 twice your Constitution score, but only if you do nothing other than take move actions or free actions. If you take a standard action or a full-round action (such as making an attack), the remainder of the duration for which you can hold your breath 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 DC 10 Constitution check every round to continue holding your breath. Each round, the DC for that check increases by 1. If you fail the Constitution check, you begin to drown. The DC for the Swim check depends on the water, as given on the table below.
* You can’t take 10 on a Swim check in stormy water, even if you aren’t otherwise being threatened or distracted.
About This Section Optionally, a character who reaches 5, 10, 15, or 20 ranks in a skill unlocks various bonuses and abilities unique to that skill. The unchained rogue uses these rules extensively, but others can gain access to them with a new feat.
In this system, characters unlock additional abilities when they attain 5, 10, 15, and 20 ranks in a skill. The skill unlocks system interfaces with the unchained rogue to make the rogue the true master of skills.
Skill unlocks give characters new abilities and ways to use their skills upon reaching 5, 10, 15, and 20 ranks in a skill. Any character with the Signature Skill feat can earn skill unlocks for a single skill, and they are a prime feature of the revised version of the rogue who uses her rogue’s edge ability to gain skill unlocks for several of her most iconic skills. Alternatively, you might make skill unlocks a universal part of the game, but you should be aware they add significant power and flexibility to skills, so giving them for free to all classes would grant power boosts to other highly skilled classes such as the investigator and bard, particularly in comparison to the rogue. Another alternative is to eliminate access to the Signature Skill feat, limiting skill unlocks to rogues and rogues alone.
With sufficient ranks in Swim, you earn the following.
5 Ranks: You gain a swim speed of 10 feet, but only in water with a Swim DC of 15 or lower.
15 Ranks: You ignore the penalties for using slashing or bludgeoning weapons underwater, as freedom of movement.
See this skill in FastPlay format.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.