Although I have not done a lot of checking for this, one good use of a thermometer is to find springs. This is best done either in the winter or the summer, when the temperature difference between spring water and surface water is the greatest. The locations of these springs will be refuges for fish when the water elsewhere either gets too warm or too cold, depending on the season.
And wherever the river goes, every living creature that swarms will live, and there will be very many fish. Ezekiel 47:9