Back in the '60's we use to regularly use a device called the "Depth-O-Plug". A water pressure tube you lowered to the depth you wanted, then retrieved it to read the temperature down there. Not sure if it's still made or not? But it did work just fine. However, after many decades, I've learned to "relate" the surface temperature, within the normal seasonal changes, to what the fish are feeling (with regards to feeding activity). Seems to work fairly well for me.
"A voyage in search of knowledge need never abandon the spirit of adventure."