Question

why is it easier to lift a heavy stone under water ?

Answer 1

because force of buoyancy apply on stone.
let stone mass=m kg
so weight =mg N
but in water
weight of stone =(mg-X)N
where X is force of buoyancy
here we see why is it lift stone under water.

Answer 2

According to Archimedes principle, a body when immersed in a fluid shows an apparent loss of weight which is equal to the weight of the fluid displaced by the body. So a heavy stone when under water, is acted upon by upthrust due to water , which ultimately reduces the true weight of the stone.