Question

why is it easier to swim in seawater than in river water

Answer 1

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 in water, as it is in ocean water, that dissolved salt adds to the mass of the water and makes the water denser than it would be without salt. Because objects float better on a dense surface, they float better on salt water than fresh water

hope it helps