The swimming skill is an automatic skill (in use automatically when needed, like the dodge and parry skills.) It allows one to swim across bodies of water (or other liquid, with the proper protections), and expert swimmers have the advantage of being able to make deep dives without the aid of spells. A character with the swim skill will not need to carry around a boat, and may not need waterbreathing for underwater dives. The higher your swimming skill, the faster you will be able to move through the water.