Cortland Depth Thermometer
It's well known that fish will move up or down in the water to stay at their preferred temperature range. Using the DepTherm Depth Thermometer you can check the water temperature at different depths to discover where the fish are most likely to be feeding. The packaging includes full instructions for use.
This clever little product will let you measure the water temperature at various depths below your boat. Simply lower down, leave for 30 seconds and bring back to surface. The DepTherm will reveal the water temperature and the depth at the which the temperature was measured. To reset, just turn it upside down, press the release valve and shake out the water.
The Deptherm includes a chart (in F) of the preferred water temperature for Brown Trout, Rainbow Trout, Salmon and Sea Trout (amongst others).