Water skiing is a surface water sport in which an individual is pulled behind a boat or a cable ski installation over a body of water, skimming the surface on two skis or one ski. The sport requires sufficient area on a smooth stretch of water, one or two skis, a tow boat with tow rope, three people (depending on state boating laws), and a personal flotation device. In addition, the skier must have adequate upper and lower body strength, muscular endurance, and good balance.