Alcohol is a diuretic, a veritable master of dehydration. So, it's surprising that some researchers actually disagree on whether or not beer is a suitable replacement for water. Initial reactions tend toward, "What? Of course they aren't interchangeable." We owe it to ourselves to hear both sides. And then, we get to talk about the awesomeness that is beer.
There are the two main questions that must be addressed: how long could a person live drinking just beer, and can it truly replace water? Jeremy Singer-Vine from Slate discussed the first question a couple of years ago, and the answer is that you'll live just long enough to develop scurvy, but "not more than a few months, probably." Blast! Our plans for ditching the water fountains at DVICE headquarters for a dozen or so R2-D2 kegerators are dashed.
Now, what about beer as a substitute for water? Singer-Vine thinks not, but Professor Manuel Castillo from the University of Granada conducted an experiment to test the claim. He had participants exercise, and certain individuals were given water, while others were given beer. "What we found is that rehydration with beer with a 4-5% alcohol level in a moderate amount, 660 ml (a little more than a pint), is not better, not worse than rehydration with water.” Hmm, not better, but not worse either.
A 2011 study published in the journal Medicine & Science in Sports & Exercise concluded that beer had numerous health benefits for athletes, but they used non-alcoholic beer in the study for some silly reason, so that one's pretty irrelevant.
Finally, The Journal of Applied Physiology wrote this about beer in a 1996 issue with a paper entitled "restoration of fluid balance after exercise-induced dehydration: effects of alcohol consumption," with results that were generally in line with Professor Castillo's experiment.
Obviously, these studies aren't exactly conclusive and you should probably stick with water, but the research in favor of alcohol does make this beer race seem a little less ridiculous.