According to Live Well by Jillian Michaels, water treading is good and efficient exercise that can burn around 300 calories per half hour session. Working out in water provides the muscles with continual resistance without putting them under undue stress.
Live Well suggests finding the appropriate depth of water before the start. Ideally, the water level should be around the shoulders, when the feet are flat on the bottom. Once there, the workout consists of lifting the knees, and then moving the arms backwards and forwards while kicking the feet. According to Live Well, the exercise can be made more difficult by moving to the deep end of the pool and moving under the water in a running motion.