Bodies of water are made up of layers, determined by temperature. The top surface layer is called the epipelagic zone, and is sometimes referred to as the "ocean skin" or "sunlight zone". This layer also interacts with the wind and waves, which mixes the water and distributes the warmth throughout. At the base of this layer is the thermocline. A thermocline is the transition layer between the warmer mixed water at the surface and the deep cooler water below. It is relatively easy to tell when you have reached the thermocline in a body of water because there is a sudden change in temperature. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.
Original article: What is a thermocline?