When the legendary ancient scientist Archimedes was asked to determine if his king's crown was made out of gold, he discovered the principle of buoyancy. The buoyant force is what holds ships up on the surface of the water, even when they are large and made out of materials that are much heavier than water is. The buoyant force is created by a number of different factors, each of which when altered, produces a different level of buoyancy.
One of the primary causes of buoyancy is the fact that water has weight. The further you go below the surface, the more water there is above you. This means that the further down you go, the more the water weighs. The greater the weight of the water, the more pressure it exerts on anything in it. Any object submerged, or partially submerged, in water has its bottom at a different depth than its top. This means that the bottom of any object has more pressure on it than the top does. The net result of this is that the water pushes upward on the object. Putting an object at different depths exposes it to different buoyancy. This is why a beach ball on top of the water just sits there, but one held far under water and then released shoots to the top like a rocket.
The size of the object in the water is another important cause of the exact form the buoyant force takes. As Archimedes observed, the object in water become lighter by an amount equal to the weight of the volume of water it displaces. If a gallon jug is submerged in water, then it becomes lighter by an amount equal to the weight of 1 gallon of water.
The density of the submerged object also determines the buoyant force's nature. If the object is less dense than water, then the buoyant force pushes it up. If the submerged object is denser than the water that surrounds it, then the buoyant force does not make it light enough to float. The object then sinks, increasing the buoyant force on it the deeper it goes.
The shape of an object also determines the nature of the buoyant force upon it. Because the buoyant force depends on the pressure on the submerged object, different areas on the bottom of the object result in different buoyant forces. If the bottom of an object is very narrow, there is little area for the pressure to push up on; therefore, there is little buoyant force.