You can run in the water without any pounding on your feet or joints. Secondly, the water is viscous or thicker than air. As you move your arms and legs through the water you encounter a tremendous resistance. This resistance can be used to repair and strengthen muscles and joints. The faster you move your arms and legs the greater the resistance you will encounter and therefore the harder the workout and the greater the strength gain.
Water exerts greater pressure on our bodies than air does.
Plus running in the water allows us to do more exercise bouts with less recovery time than a similiar workout done on land. The water pressure coupled with the lack of gravity acting on us and water's cooling affect allows for faster recovery from strenuous exercise. Blood flows back to the heart faster and the body is cooled quicker in the water than on land. Any device which keeps your body afloat and in an upright position will work. Products such as the Wet Vest and Aqua Jogger are specifically designed for deep water running but almost any flotation device will work from a ski belt to a life preserver.