Question

why it is easier to sail a boat in seawater than in river water

Answer 1

because river water is running water whereas sea water is a still water

Answer 2

Sea water contains many dissolved minerals and ions that cause it to be more dense than fresh water. Because of this, objects sea water exhibit greater buoyancy in sea water than fresh water (Objects displace an amount of water based on their volume but the buoyancy is determined by the weight of the water displaced). As a result, a ship will float higher in sea water, reducing the drag of the ship against the water while sailing. This effect of heightened buoyancy is also the reason people float so easily in high-salt lakes such as those in Utah and the Dead Sea.