Question

why centre of pressure is below centre of gravity

Answer 1

Centre of pressure is the point at which the resultant force of buoyancy acts,while centre of gRavity is the point at which the resultant gravitational acts.(actually at these points the force doesn't acts alone ,but we choose a such point where the effective force can be assumed)

Centre of pressure of a body immersed in a fluid depends on the physical dimensions of The body,(ie. the shape and the volume displaced by the body).

So it's dependent on the body you take,that whether the Centre of Pressure will lie ABOVE or BELOW the centre of graviTy