How a Small Density Difference Helps You Float
A body in water is supported by the water it displaces. As you sink, more of your body goes below the surface, displacing more water and increasing the upward push. Floating becomes possible when that push balances your weight. Because salt water is denser, a partly submerged body displaces a heavier amount of it than the same volume of fresh water—so the balance can be reached with less of the body underwater.
Your Body Still Sets the Balance
People are not solid blocks: lungs, fat, muscle, and bone differ in density. Air in the lungs makes the chest more buoyant, while body composition affects how much of the body tends to sink. That is why two people in the same water may float at different heights, and why even one person may float more easily after taking a relaxed breath.
Salt water does not lift a person independently of their size or weight. Its advantage comes from the weight of the water displaced, and the difference depends on how salty the water is. In practice, a person may need to submerge less of their body to stay afloat—but still has to keep their airway clear and cannot assume that floating alone makes unfamiliar water safe.
