Swim

Swim:

Usage: automatic

The swim skill is available to all classes and is specifically used to allow
passage over watery terrain (deep water). It is possible to fly over this type
of terrain which does not require swimming. However, it should be noted, even 
when you are flying over a watery terrain, your ability to swim may improve. 
The reasoning for this is simple.

As you grow more familiar with a particular type of action, whether performing
it yourself or watching others perform it, you can improve in your ability to
perform the action. In succession, it is possible to improve your ability in
some feat, not only by performing the event, but by being exposed to the
aspects of the feat itself (in this case being exposed to the specific terrain
type). Therefore, it is argued that by merely flying over the terrain type you
can improve in your ability to traverse it by understanding its architecture 
from a distance.

Keep in mind, swimming is difficult and takes a lifetime to fully master. For 
more information see 'help climb'.