Hydrostatic Weighing measures body fat underwater. It is an accurate method of measuring body fat (2-3% margin of error), but the test can be uncomfortable and frightening.
The Hydrostatic Weighing test involves the individual sitting in a tank of water and blowing out as much air as possible. Once dunked in the water the individual continues to blow more air out. As fat is lighter than water the more fat the individual has the more they float. The scale underwater then measures body density.