Most fishermen realize the importance of water temperature on fish and insects. There are numerous charts that detail the optimum ranges. Strangely, few fishermen bother to carry a thermometer. Some time ago on a trip I asked 5 experienced anglers if they had one. The answer was no. Then I asked if they would each guess the water tempetature. None of them came within 5 degrees of the correct answer.
Over the last 20 years I’ve owned lots of thermometers but sooner or later I had to replace them due to breakage or failure. That changed some 3 years ago when I bought the Fishpond Swift Current. It’s a laboratory-grade unit protected by aluminum case which takes all the abuse I dish out. The current model at $18.95 can measure water temperature in both Celsius (10 to 50 degrees) and Fahrenheit (20 to 120 degrees). It's always attached to a D-ring on my fishing vest and ready for use every day of the year.