Water is not very accurate as ir light for the gun refracts and reflects from shiny objects ideally you would use a target of some sort a bit of brown paper perhaps not sure that would work however.

The first part is check the specs, it is almost certainly +/- 2ºF. The best test is place a know accurate thermometer or thermostat probe of a brown paper bag (non reflective and the calibrated ε. Herpstats are quite good I have personally checked with a high end thermometer 19 probes and the best was 0.2 off and the worst was 0.7 off. Most being in the 0.5 area. Well inside the specs for the unit. (+/- 0.9ºF) It is actually quite inexpensive as a sub one degree thermometer often thermometers of this accuracy cost more than the basic 100$

IR guns are great tools but they have a lot of quirks to keep in mind. I would expect most to behave inside the specs unless pointed at a shiny object. Accuracy is a slippery term some say 4º up or down from correct is accurate and other accept 6º up or down from correct as accurate. For me one degree up or down I can live with (just) anything outside is not accurate. To me IR guns give a very rapid 'feel' for what the temps are like and the shape of the gradient but are not precise accurate tools just a quick rough over view.