Swimming is probably the closest you can get to a perfect form of exercise. It is non-weight bearing and places no stress on the bones and joints. The combination of buoyancy and resistance of the water makes this sport ideal for working out.
Safe on the joints
Compare swimming to running: Runners' legs and feet have to withstand a gravitational force that is two to three times their body weight each time their foot strikes the ground. Swimming, on the other hand, places almost no stress on the joints, while having the ability to provide as intense a workout as running. Each movement in water requires greater exertion because of the need to overcome water resistance.
Great for rehabilitation
Working out in the water is also a great way to recuperate from a sports injury, recover from surgery, or even to keep your fitness level sustained while you need to wait to get back to any sports training.