This mod adds the swimming skill (separate from fitness). Now, be careful with this mod. If your Sim doesn't know how to swim or has a low level (swimming skill level 0-2), they can easily drown if you spend too much time in the water! It also includes swimming in the ocean if you have Island Living, so don't go too far out at first.
New SWIMMING Certification trait
New interaction for Throwing a Lifesaver
Changes to Island Living careers: Lifeguard and Diver
Changes to swimming classes and interaction with trainers.
Anyway, it's worth it
This is just the translation into Spanish, I leave you the link to read the details and download the mod