As a singer I have been asked what is the best drink for vocalists. Well, my answer is water. Plain and simple...water! A lot of people like to put lemon in their water but in my opinion and experience lemon is too acidic, it tends to dry you out more that hydrate you. This is similar to white wine, for me, I totally lose my voice so that is also off the list. Some people like to drink ice water, well, that is really an unkind thing to do to your cords. Room temperature water is actually the best! If you really want to put something in you water, pineapple juice; pure, not from concentrate, seems to work.
I often drank coffee on a gig but honestly that was when I was younger and seemed to think I needed some sort of stimulus without losing control of what I was doing. I was just fooling myself at that time. When I had a sore throat, or a cold coming on and I had a show, I would be back in the dressing room madly steaming myself and gargling with Cognac. I have to say it was soothing, but you don't want too much of a good thing.
Seriously people, water, water, water! The very best hydration therapy for singers!