Why does a plastic ball released under water come up to the surface of water?
The density of a hollow plastic ball is less than the density of water due to the air in the ball. So, the weight of the water displaced by the ball is more than the weight of the ball. Thus, by the principle of floatation the ball floats in water. Even if you push the ball into the water the buoyant force will push it upwards and keep it floating once you release your hand.