Check out this graph from the Economist showing the ratio of water use to drink/food produced. According to this data, taken from the new book The World’s Water 2008-2009 by Peter Gleick et al. (Island Press, 2009), coffee requires an astounding 1,120 liters of water per liter of coffee produced. Compared to that, 300 liters seems like a paltry amount for making a liter of beer. (Keep in mind this includes upstream inputs too, not just the water used in production at the brewery.)
How Much Water Is Required to Make Beer?