Buoyancy

Upthrust on body = weight of fluid displaced by the body
This is known as Archimedes principle.

If the body is immersed so that part of its volume V1 is immersed in a fluid of density r₁ and the rest of its volume V2 in another immiscible fluid of mass density r₂,
Upthrust on upper part, R1 = r₁gV1
acting through G1, the centroid of V1,
Upthrust on lower part,R2 = r₂gV2
acting through G2, the centroid of V2,

Total upthrust = r₁gV1 + r₂gV2.
The positions of G1 and G2 are not necessarily on the same vertical line, and the centre of buoyancy of the whole body is, therefore, not bound to pass through the centroid of the whole body.