Proper depth to take water temperatures depends on what questions you are trying to answer.
Most temperature profiles are done to determine the location or thickness of the epilimnion, thermocline or hypolimnion. Or temps will indicate if a water body is stratified. Temps can indicate location of spring fed areas.
Some take temperatures to help in determining when fish will be spawning. Some fishermen take temps to find where fish might be located.
Surface temps in early spring will indicate how active fish could be and if they may be bitting or how fast to move the lure/bait.
If one knows their fish biology, temps will fine tune your annual calendar periods: pre-spawn, spawn, post spawn, pre summer, summer peak, summer, post summer, fall turnover, cold water, frozen.
