While KSC does use a lot of water for a launch, as CodingDuckling's answer points out, I'd have to say that, for comparison purposes, the answer is "much less than a fish farm." According to USGS, as of the year 2000, the state of Idaho alone used 1,970,000,000 gallons of water per day for fish farming. By comparison, NASA's 300,000 gallons per launch isn't as impressive of a figure. Also according to USGS, the U.S. as a whole used about 355,000,000,000 gallons of water per day, which is over 1,000 gallons per day per capita. So, an Apollo launch from KSC only used about as much water as is used per 300 people in the U.S. on an average day.
As disappointing as it may seem, I'd honestly be pretty surprised if the average orbital launch didn't use more water for the bathrooms and drinking water for all of the people involved for the years of work involved to make that launch happen than is used for the actual launch itself and other purposes directly related to the rocket itself.