Please follow the link to read an article from the International Journal of Aquatic Research and Education, 2010, By Human Kinetics Inc.
There research shows that exercising in water and specifically using an underwater treadmill can benefit those with Osteoarthritis.
They concluded that;
"patients diagnosed with OA will display similar energy expenditures during short-term exercise on an underwater versus land treadmill when speeds are greater than preferred. This finding along with the perceived pain findings would indicate that patients with OA may receive the same aerobic conditioning during underwater treadmill exercise with less joint pain than performing the same exercise on land. While future longitudinal research is needed, underwater treadmill exercise may also lead to greater improvements in mobility when compared with the same exercise performed on land."