This is something that scientists and surfers refer to as wave breaking. It is very complicated and very important to understand, not only for surfing. When waves break they mix air and water together. Breaking waves can pull sand up from the beach and move it around. This is important for any of the creatures that live or play in the ocean, including people. Scientists do not completely understand how waves break yet, and very powerful computers are needed to study them in detail. What we do know is that waves break when the top of the wave is moving faster than the bottom of the wave. This can happen when wind is blowing on the waves, pushing their tops forward and knocking them over. It can also happen when waves come in to a beach. The wave gets very steep, and the top of the wave comes flying forward, sometimes crashing all the way to the water below.

The ocean 'turns over' due to density differences between different masses of water. If you have water at the surface that becomes more saline (due to evaporation of freshwater, for example) its density will increase and if it increases enough to be more dense than the water below it, the surface water will sink.