The only place you would do this and see it is in the brewkettle. If the water is boiling it is about 212deg. If it is not boiling it is under 212 deg. If there are a lot of small bubbles and turbulence the water is approaching 212 deg. Why keep a thermometer floating in it? It seems like just one more thing to go wrong. There are enough things to go wrong without adding extra. I have a long probe thermometer I use by hand and a laser thermometer for quick checks. I own a floating thermometer that I have never used because I can't bring myself to float a glass bulb in my wort.