Swimming helps avoid two of the potential causes of joint pain. Moving the knee and actively exercising promtes muscle strength and is good for the fluid that cushions the knee. It is also an exercise that does not place weight on the joint and therefore does not cause damage in that way. Swimming is considered one of the better ways to exercise  your legs without causing damage.

While swimming won't save your joints from certain problems you may have but it can help.  Swimming being one of the best cardiovascular workouts around without having any impact on your joints.  Unfortunately, life still impacts our joints and there is nothing we can do about but we can make it easier.  So switching up your rountine from impact type of workouts like running on pavement to swimming can help but it won't be a cure.

Unless you're a mermaid or have the last name of Phelps and spend all day in the water, swimming won't unconditionally save you from getting joint pain, but it may help delay the onset. And it will certainly help you stay in shape while you're recovering from problems associated with your joints.