Not all people are able to float easily. People have different buoyancy and the combined buoyancy of materials in our body makes this a matter of physics. A person who has a higher percentage of body fat will likely float better than a person with a lower percentage of body fat. Fat is lighter than water and more fat will increase the overall amount of buoyancy that a person has. It is as simple as thinking about a child floating with their inner tube on and then sinking like a rock if it is taken off.
