The buoyancy of water reduces the “weight” of a person by about 90%, which means that the stress on weightbearing joints, bones and muscles is similarly reduced. For this reason, it is unlikely that a water workout will result in injury or leave you with sore muscles. That’s why the pool is such a great place for people with arthritis or back problems to exercise. It is also great for those who are new to exercise.

American Council on Exercise June 2009