Swim is the skill used to tread water and moving through water for non-aqautic (or non-amphibious) creatures. Swimming is a more natural exercise than most give it credit but a person that does not have levels in swim and finds themselves in water deeper than what they can stand in, or fast moving water must first save vs fear at the difficult level (31) or acquire the feared and drowning status. Creatures immune to fear can ignore this roll as well as skilled swimmers.
Treading water to keep from drowning is a test of conditioning, knowledge and, encumberance. Swim levels aid the user when trying to stay afloat and breath properly while in the water. Movement is restricted to your step and you do not have any double move options....so no charging.
While in water your gear absorbs it and gains weight. For every 10 lbs of gear add 1 lbs. Figure your encumberance from this and adjust PAP cost accordingly.
Treading water is based on 3x your Health score in minutes + 10x Swim levels additional minutes. PAP cost is 0 + any encumberance adjustment paid every 10 minutes.
While in water you may also have to deal with the hypothermia condition if the water is to cold.
For every 4 levels of swim you may increase your move by 1.