It’s a long time since I did any physics, but it should be possible to graph out the water temp at different depths knowing the temperature at the surface. My own feeling is if you took the water temp at 2ft it’s not going to be that much different at 6ft. What I was thinking is, fish being cold blooded will seek the water temp that’s the nearest to their optimum body tempfor maximum efficiency. However, the need to feed may take them out of this comfort zone if food is easier to find in the colder water.