Water aerobics or “waterobics” is the performance of aerobic exercise in shallow water such as a swimming pool. In some areas it is known as AquaFit or “aqua aerobics”, and is a type of resistance training.
In addition to the standard benefits of any exercise, the use of water in water aerobics supports the body and reduces the risk of muscle or joint injury. The mitigation of gravity by flotation places less stress on the joints when stretching, and can allow a greater range of motion. The mitigation of gravity makes water aerobics safe for individuals able to keep their heads out of water, including the elderly Exercise in water can also prevent overheating through continuous cooling of the body. Most classes last for 45-55 minutes. Another benefit is that you do not even have to be a strong swimmer to participate in water aerobics.
http://www.youtube.com/watch?v=BPh5GuNn11w Here is a link to some examples of this kind of exercise!