An expert (how expert I could not say) on the radio the other day was mentioning that "of course" the water in coffee and even beer was acceptible as part of the daily water intake. He was dismissive of the idea that it was not. I think this was something to do with the fact that most of the water is to help our waste-disposal systems, and diuretics just speed up this process, don't harm it. This fits with the data mentioned by Wayne suggesting that our basic water loss can be replaced by the water in our food. Keeping hydrated then allows us great water throughput to take away bodily waste, whether that water contains diuretics or not.
Anyway some of the data should be available to pilots or at least aviation medics. My father was involved in the sixties in experiments on dehydration, sitting for an hour at a time at high alert in a Vulcan on a hot day the crews then had their water loss measured.