To me, as long as you're drinking a bottle of water with it I see no harm. You get hydrated from coffee. Of course you need to drink water in general, but the concept of drinking cups of water to balance out dehydration from coffee is just a myth.
Reptiles do not maintain their own body temperature within a narrow range like mammals and birds, so controlling their environmental temperature is critical for normal activity and good health. Reptiles are poikilothermic-which means that their body temperature alters with the surrounding environmental temperature.
In very hot weather (say, over 100 degrees F), they can easily overheat and die if they cannot find a cool place (such as a shaded rock or bush). This is called behavioral thermoregulation: the snake can't internally moderate its body temperature, but it selects sites that will help it do so externally.