One thing is the definition of the surface layer of water (0 to -20m correct) and other how to measure its temperature. If you use a floating thermometer the s

One thing is the definition of the surface layer of water (0 to -20m correct) and other how to measure its temperature. If you use a floating thermometer the sun-rays are probably warming it or the winds are cooling it, so you are including an error in your sample. The results are higher or lower water temperatures than the real ones. It is assumed that the water layer mix uniformly in the first meters by wave forces and wind so we can measure surface at 10m perfectly