What do you mean by buoyancy? Why does an object float or sink when placed on the surface of water?
When a body is partially or wholly immersed in a fluid, it experiences an upward force. This upward force is the buoyant force (or buoyancy).
Forces acting on the body inside water are buoyant force and weight of the body. If the buoyant force is greater than the gravitational force then the body floats, otherwise it sinks.