Swimming is a skill used to swim on water tiles. It can be a dangerous practice to people with a really low skill, as it is fairly easy to just stop moving as the character is drowning. After three or four times the player cannot move, it usually dies.
For this reason it's advisable to swim on the coast to train the skill, and start using it as a boat alternative to do some Passive Fishing, or just take some Njerpez by surprise, after the player has reached 50-60 % proficiency with it.
With the temperature update and the introduction of hypothermia, water is deadlier than ever. Even once the ice melts away, deeper water can retain some of the deathly chill of winter, making swimming risky unless one is prepared to strip and wait by a fire afterwards to dry and warm up.