In what water and why is it easier to swim in sea or river?

In water, the buoyant force of Archimedes will always act on a person, which allows a person to stay afloat. It depends on the type of fluid (density), the acceleration of gravity and the part of the body’s volume submerged below the surface of the fluid. The last two factors are the same for any water, therefore, it will be easier to float in a liquid that has a higher density. And if river water has a density of 1000 kg / m ^ 3, and sea water – 1025 kg / m ^ 3, then it will be easier to swim in sea water.