Buoyancy in water can be due to a number of factors. The biggst one being the difference in physical makeup between men and women. When swimming the heaviest part of our body is our pelvis which is why we generally more inclined to be in an up right position in water. (Generally for people who rarely swim).
Being the child bearers or the species, women tend to carry more fat around our pelvis and thigh area and because we have wider set hips we tend to float better.
Cultural differences can also come into play here as different ethnicities have a different type of genetic makeup. People of an Asian background are more likely to be narrower whereas people of a European background can be found to be more fuller figured.
To help with buoyancy all you need to remember is that because our pelvis is the heaviest part whatever we do to our head will affect our leg posionting and vice versa. A good head position for say freestyle would be to have your head under the water not just your face.
NOTE- I have tried my best to word this as good as possible but my apologies if I have offended anyone in my answer.
Answer BYes, women do float easier then men because the women have more fat tissue than men and have a better distribution of it, their bones are less heavy and smaller, and their muscles are more flexible and lightweight.
Answer CActually, for a man and woman who weigh the same amount, they have identical fat, bone, and muscle density. They just won't have the same amount of each. The prime difference is in percentage of body weight that is fat, as fat is significantly less dense than water (both muscle and bone being more dense than water). Even a woman in good shape will have at least 10% body fat, while a comparable man will have 5% or less.
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