Insulin Resistance, Body Weight, Obesity, Body Composition, and the Menopausal Transition (ScienceDirect, 2000)
Because bone and muscle have greater density than water, a person
with a larger percentage of fat-free mass will weigh more in the
water. Conversely, a larger amount of fat mass will make the body
lighter in the water.
The density of:
- Water (in the swimming pool) = 1.0 kg/L (source)
- Adipose tissue (body fat) = 0.92 kg/l (source)
- Muscle = 1.06 kg/L (source)
- Bone (cortical) = 1.85 kg/L (source)
Things that are less dense than water will float and those that are denser will sink. So, who may sink in water even without breathing out air from the lungs: the one with little body fat, a lot of muscles and strong (thick) bones. To know "how many people" would sink, you would literally need to measure their body density (by underwater weighing or hydrodensitometry - see the first link), because it does not seem realistic to calculate this. Height/weight ratio and body fat percentage alone also do not help.