Swimming is a skill that, much like foraging, isn't very important compared to others. At least, we don't think so.
Swimming is a skill of which it is increased by doing what it says, swimming. The higher level, the longer you can swim for.
The activity itself is a way of getting between islands without a boat, it is the least safe way to get from island to island. It can only be used in the ocean, counting that flowing into caves, the water in the Plateau island cave is unswimmable for an example. So are any ponds.