Swimming

The human body is composed primarily of water, and it has a very similar density to water. Roughly, 70% of the body is water; while the lungs are filled with the air, the body is slightly less dense than the surrounding water, which exerts a buoyant force on it. Thus staying afloat requires only a slight propelling of water downward relative to the body, and transverse motion only a slight propelling of water in a direction opposite to the direction of intended motion. This propelling is accomplished by using the hands and forearms as paddles, and by kicking the legs to push water away from the body (though kicking accounts for relatively little overall). Since salt water (e.g., the ocean) is denser than fresh water (e.g., most swimming pools), less effort is required to stay afloat in salt water than in fresh water.
A number of swimming styles have been developed based on the implementation of some or all of the following principles:The torso and the legs should be kept as parallel as possible to the surface of the water. Dropped legs or a slanted torso dramatically increase drag. The hand should be extended forward of the head as much as possible. This increases the average length at the water-line, substantially increasing speed.