http://www.spine-health.com/wellness/exercise/pool-therapy-relieve-osteo...
A pool or tank of warm water provides an ideal environment in which to exercise because its buoyancy counteracts gravity, thereby decreasing the weight placed on painful joints and the spine. In fact, when immersed to neck level, buoyancy supports 90 percent of the body's weight. In waist-depth water, buoyancy can still support 50 percent of body weight. Diminished weight bearing stress is one of many of the advantages of pool therapy for patients with osteoarthritis.