Why does a block of plastic realeased under water come up to the surface of water?

Any time a mass is less dense than the liquid it is in, it will float. Said another way, when a volume of anything weighs less than the weight of the same volume of the liquid, it will float in that liquid. This applies to all fluids and gases, including why a hydrogen or helium balloon rises in the atmosphere, and how a submarine floats or sinks by altering the amount of weight on board (by intaking or expelling water, while its volume is a constant.) The plastic floats because it is less dense than the water (less weight compared to the same volume of water).