Any body in water floats or sinks depending on the buoyant forces that push it up and the gravity that pulls it down. If the body is heavier than the water it displaces, it sinks. If the water the body displaces is heavier, then the body floats.
Archimedes' principle states "any body fully or partially submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced." Thus, if a rock weighing 10 pounds displaces 3 pounds of water, it sinks. However, if a human body, weighing 150 pounds displaces 140 pounds of water, the body floats.
Physicists use density as a method of computing the mass per volume of an object. Another way of stating Archimedes' principle is articles denser than water sink while those less dense than water float. The air one holds in the lungs makes him less dense and more buoyant due to the increase in volume of the ribcage. Floating or sinking is also dependent upon the fluid in which one attempts to float. Thus, humans find it more difficult to float in fresh water than in salt lakes, such as the Great Salt Lake or the Dead Sea. The dissolved salt causes the water to become denser, so the volume an individual displaces has a greater weight in salt water allowing him to float with ease.