Let initially the cone be in equilibrium i.e. floating

From the diagram, it is clear that
Buoyant force weight of the cone.

Now, the cone. is depressed by small distance and released it executes simple harmonic motion. In simple harmonic motion, the buoyancy force acts as restoring force which is given as

Since, the displacement is very small, the excess submerged shape of cone. can be considered as a cylinder whose volume is given by .

Thus, force is equal to restoring force, hence

or