why is it easy to swim in sea water than in river water?
when a person is floating on the water the its apparent weight is equal to,
= true weight of the person( Mg ) - weight of the liquid displaced( Vρg )
M is the mass of the person.
ρ is density of the water.
So, if the density water is more then the apparent weight of the person is less. therefore it easy to swim in the see water.