It is perhaps more accurate to say that floating, rather than swimming, is easier in salt water. This is entirely due to the density of salt water in comparison to the density of outdoor fresh water or indoor pools. A swimmer's physiology also affects his ability to float in any form of water.
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Archimedes said that an object partly or completely immersed in fluid is "buoyed up by a force equal to the weight of the fluid displaced by the object." Therefore, heavier, or denser, water produces more force to keep you afloat. Fresh water weighs 62.4 pounds per cubic foot compared with salt water, which weighs 64 pounds. Salt water itself varies in density, and according to open water swimmer Nuala Moore, a combination of salt density and deeper water contributes to increased buoyancy for swimmers. Moore also points out that swimming in salt water is easier because the body is higher in the water, allowing the swimmer to surf and glide, whereas swimmers whose bodies are lower in the water have to exert more push to move forward.
Swimmers with a lower specific gravity have even more buoyancy than others in both salt water and fresh water. Women typically have a lower specific gravity because they have more body fat than men. People with a higher proportion of bone and muscle to fat are more likely to sink than swim.