There are many different types of swimming which arise out of the basic techniques. You know how to float, tread, breathe, and move properly through the water. With these techniques, you can escape currents, dive properly (and beautifully if combined with Acrobatics), and study the depths of the ocean.
If combat occurs in the water, every 3 or 4 levels of this skill could offset one combat penalty incurred by the act of swimming. This is at the GM's discretion.