The centre of gravity of a floating object.They intersect at the center of flotation of the even keel waterline....
The centre of mass and centre of flotation may be in different places, which would cause the rotation effect that Shambolic spoke of.
Using the centerboard as a lever and the center of flotation in the hull as the fulcrum, the lever is longer in the older boats than in the newer ones with the flotation in the seats.