The water temperature is the mean temperature of the seawater under normal conditions, at a situation in which cables are installed, or are to be installed.
Vertical stratification definition in bodies of water creates zones, including the thermocline, based on differences of temperature, salinity and density. For layers determined by temperature, the top surface layer is called the epipelagic zone. This layer interacts with the wind and waves, which mixes the water and distributes the warmth. 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 cooler deep water below. At the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.
In the ocean, the depth and strength of the thermocline vary from season to season and year to year. It is semi-permanent in the tropics, variable in temperate regions (often deepest during the summer), and shallow to nonexistent in the polar regions, where the water column is cold from the surface to the bottom.
The sample equation is an example of a thermocline below the epipelagic zone and therefore only valid for depths about >200 m. The equation was taken from the paper 'Correction of Single Log-Derived Temperatures in the Danish Central Graben, North Sea' by Douglas W. Waples, 2004