I love the infrared thermometer guns for solid radiating objects around the house or jobsite. They are certainly easy to use, especially to measure difficult to access areas.
However, I have never heard of anyone using them for pond temperatures. I think(?) a surface measurement at a pond might be heavily influenced by the evaporation rate, so you may be measuring pond temperature plus wind speed plus relative humidity, etc.
I have seen BG spawn in as little as 6" of water. However, if I was trying to observe the BG each year, I might create may own "standardized reference" for my pond. Maybe 2' deep off of the end of your dock? After a couple of years of data, you might be pretty accurate as to when you need to start looking for BG on the nest.
Water temperatures AND photoperiod control the spawning of many fish species. Since BG can spawn multiple times each year, they may not be quite as sensitive to photoperiod to start things off in the spring?
Hope that helps with some ideas behind your question.
