Last year I bought a AquaVu DT-100. One interesting thing I learned was that the water temps (during summer) stay within 1 degree all the way to the thermocline, sometimes down to 20'. One Garmin (trolling motor) reads the same temp as the DT-100 while the other Garmin (transom mount) reads 2 degrees different. I figure temp scales are man made and can be any figure so if the scale goes up it is getting warmer and etc.