I saw a performer at Whitby drink a glass containing 10 double gins and lime before he went on stage. His act was ok - although a bit unsteady on his pins
A friend told me this same performer was so drunk at a gig that he vomited during a song and carried on as if nothing had happened!
I have been known to drink a glass of beer myself - on special occasions, and it does nothing to improve my singing. However, not having a drink does nothing to improve my singing either!
I think it all comes down to moderation, especially if you are a professional singer.