The Center of Gravity is the point through which the gravitational force acts (usually in the region between the points of the hips when the body is in the anatomical position). Its magnitude is described as “weight”. The position of the body segments determines the actual site of the Center of Gravity.
Centre of Buoyance
The Centre of Buoyancy is the point through which the buoyant force acts (usually in the lower chest area but it too is determined by the position of body segments). Some body parts are more buoyant than others, and so the centre of buoyancy usually does not coincide with the center of gravity. The centre of buoyancy relates to the body’s volume while the center of gravity relates to the body’s mass. Since these two factors normally are different, they are usually sited in different areas of the body. The distance between the centers of gravity and buoyancy usually is greater for males than females.
The divergence between the locations of the centers of gravity and flotation presents a problem for humans. In most cases, rather than floating level, the body rotates until the centers of gravity and buoyancy are aligned vertically. The body then displays a motionless float at that angle. The water supports the weight of a motionless swimmer but only at that angle. This is illustrated in the figure below. When the body commences in a horizontal (streamlined) position the relationship of the centre of buoyancy to the center of gravity will produce a rotational force and subsequent movement until the angle of flotation is attained.
(Source: Swimming science Bulletin ’07. Brent S. Rushall, Ph.D.)