Water aerobics (waterobics, aquatic fitness, aquafitness, aquafit) is the performance of aerobic exercise in fairly shallow water such as in a swimming pool. Done mostly vertically and without swimming typically in waist deep or deeper water, it 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.
Aqua aerobics requires access to a swimming pool via facilities, and in addition to any membership fees to access facilities, classes may cost extra. Although aquatic exercise greatly reduces the risk of injury, not as many calories are burned as would be in some other activities. Though aquatic activities in general expend more energy than many land-based activities performed at the same pace due to the increased resistance of water, the speed with which movements can be performed is greatly reduced.
See also 
- Aquajogging, a similar program for surgical patients