It can't do any harm. Water is basically helpful to the skin because all bodily functions take place within it (we *are* 70% water, or whatever the figure is). Whilst awake you drink at least once an hour (more, I hope). But while you're asleep you're not drinking. When you wake up the first thing you do is have a drink from the tap, right? What does that tell you? That your body's thirsty. If you're thirsty then your complex digestive system is thirsty. Give it a helping hand before you nod off and have a few gulps.
And here's a personal tip of mine that might just help your skin: don't eat after 8 o'clock. Again, think about it: if you're asleep you're not active. What happens to that late night sandwich in your belly? Some of it gets digested as normal but then you get full and the rest wreaks havoc inside you. What doesn't go to your hips comes out in your skin. It ain't rocket science