How to understand that the pressure center of the action surface in the still liquid of fluid mechanics is below the center of mass, which is roughly at the center of mass?
The center of pressure and the center of mass are two concepts, and the center of mass is related to the shape of the active surface, which is similar to the center of mass of a plate with equal thickness. The pressure center is related to the depth of the action surface from the liquid surface, and is the average action center of the force distance of the static pressure acting on the flat plate. Because the static pressure is proportional to the depth of the liquid surface, the pressure center is always below the center of mass. The distance between the action surface and the center of mass is related to the depth of the liquid surface, and the deeper it is from the liquid surface, the closer it is.