Most of the places I've lived serve water as a matter of course if there is no drought, then they have a policy of "ask for it." No one has ever begrudged me or charged me for it, but having been a waitress once, I know that we threw gallons upon gallons of water away every night. So when the area dries out, simply ask politely for a glass of tap water (if you word it other ways you may get charged for bottled water, and in some other countries, no matter what, you will get charged for it and get a bottle of water).