When one reads about fish and marine mammals, it is always said that for example blubber of marine mammals helps them with buoyancy because it is so low density and similarly for example sharks have large low density liver (possibly up to 30% of their body mass) and mainly cartilage that is about half the density of a normal bone (and it is implied this is necessary for buoyancy).
However, many terrestrial mammals like humans are already positively buoyant when having lungs filled with air (most humans in their normal weight), so it is not obvious why any kind of adaptions for marine animals are necessary as their body density composition could a priori be essentially just that of humans. There thus has to be a factor which makes it beneficial for e.g. marine mammals to have more something denser in relation to humans (like bigger muscles in relation to lower density tissue) which then needs to be compensated with such things as blubber. I am looking for what that factor is exactly.