A body is floating in water in stable equilibrium. If G be its centre of gravity, B is the centre of buoyancy and M be its metacentre, then 

In the filled position of the body (angle of lift $\theta$ is very small), the centre of buoyancy B is shifted to the new position B¹. In this case buoyant force F_B and weight W form a restoring moment which brings the body back to the original position as shown in Figure - (1). Restoring moment can be formed only when M lies above G. But B may lie above or below G.