I have a client who claims he is a "sinker" in the water. He would like to know why some people are "floaters" and others are "sinkers."
This takes me back to my swim instructor training. Several factors will determine whether one person sinks or floats (or seems to) more than another.
The first is body density and composition. Muscle, bone and fat all have different densities and therefore a different "floatability" in water. For example, two people with a similar size, or even weight, can have a different density as they would have different bone, muscle and fat ratios. (i.e., If two people both weigh 72kg, and subject A has 10% body fat while subject B has 15% body fat, all things considered, subject B would be more likely to float). In fact, this is the premise behind hydrostatic weighing, where an athlete’s body density and composition is determined through comparing the scale weight of the body versus the underwater weight of the body (corrected for the density of water). Consider the impact of "air within the body" on floatation. When in the water, air in the sinuses of the skull can be used by physiotherapists to create a traction (separation) effect for treating the cervical spine. Likewise, an immediate way of changing your body density is to breathe out and decrease the amount of air in your lungs.
Try this: Take a deep breath and attempt to lie flat on the bottom of a swimming pool. When in position, breathe out and see what happens. You should sink further.
Another deciding factor is the position of your balance point or pivot point when in the water. Consider two people, the same shape and size, floating on their backs in water. In order to balance over the pivot point, each would need to have equal water displacement on either side of the pivot point. So if one person had a more muscular upper body, their weight shift would need to move up towards the head. Whereas, if the other person was heavier around the hips, the pivot point would need to move further down toward the legs.
Try this: Float on your back in the water with your legs straight and arms at your sides. Slowly, take your arms out to the side then above your head (arms are always in contact with the water). With your hands above your head, bend your knees. You should find that your "floatability" changes with the changes in body position, just as your "balance-ability" would change when lying over the pivot point of a see-saw.
So with this in mind, whether you float or sink in the water can be influenced by your specific body density, the distribution of this density and your body position in the water at that time.