Complex sentence: However, buoyancy varies on the basis of both body composition and the salinity of the water, with higher levels of body fat and saltier water both lowering the relative density of the body and increasing its floatation.
Split into simpler sentences: However, buoyancy varies on the basis of both body composition and the salinity of the water. Higher levels of body fat and saltier water both lower the relative density of the body and increase its floatation.