Question

Why is it easier to swim in sea water when compared to river water

Answer 1

It's because the level of density is very high due to high salt concentration. ... The salts dissolved in sea water make it more dense than fresh water. This means you float higher, making it easier to swim. The Dead Sea in Israel is extremely salty which provides so much buoyancy that you don't have to swim on water.

Answer 2

when salt is dissolved adds to the mass which makes the water denser than it would be with out salt