I agree with John. Dehydration doesn't immediately go away when your cords come into contact with water. Your vocal cords might feel temporary relief when you take a swig, but after a minute or so, they'll return to that old, dry feeling.
When dealing with dehydration and the effects of it, consistent water intake becomes extremely important. I don't know how true this is, but it was once explained to me that your vocal cords become hydrated not at contact with water, but more so when there's enough water within your blood stream. So, in order to "rehydrate," it's not so much about drinking a bottle of water before you sing, but maintaining a consistent habit of drinking plenty of water each day (and potentially limiting your caffeine/alcohol intake).
I'm a professional vocal coach, so I sing more hours a day than most would argue is healthy. In efforts to ensure I'm performing to the peak of my abilities, I mostly drink water (albeit carbonated water, b/c I enjoy the bite of the carbonation). I easily drink 100ozs a day. Now don't get me wrong, I still drink a little bit of coffee for a caffeine boost, but I'll easily drink 3 or 4 cans of soda water for every cup of coffee. This is likely more extreme than what you'd need to do, but this is something I do as a singer who uses his voice day in and day out, when sick or when well... for what it's worth.