I don't know about dehydration specifically, but I have seen enough of the effects of alcohol in the outdoors to really get my blood boiling.

I became sick and tired of hauling victim after victim, often dead, out of the woods and finding that their blood alcohol level was comfortably above the legal limit for intoxication. Alcohol impairs your judgement - not only that, it makes you do stupid things.

I doubt whether the dehydrating effects of a single serving of beer would appreciably affect someone in an outdoors setting or cause any problems that could not be countered by a glass or two of H2O. And I suppose the immutable laws of physics would allow you to boil off the alcohol if beer was all you had. I would avoid all alcohol in any significant undertaking. I dimly recall some research involving marathon runners in which impairment could be demonstrated for hours after the consumption of a moderate amount of booze.

By the way, the opinion I have just stated is quite different from the actions I took in my youth. I was lucky.....