(M9) "It's just the same as drinking water... right?" - Mike

All these guys do is eat, sleep, drink, and get laid. Occasionally, they fight fires. Seeing as how they do it so infrequently, I can't say that I'm surprised that Mike (still the Probie in this episode) forgot to fill the cooler with drinking water before going out on a call. All Mike does is eat, sleep, drink, get laid, and forget stuff.