After ten years working as an esthetician I've seen many, many faces. Unfortunately I see chronic dehydration daily. Superficially dehydrated skin (easy to fix) behaves differently than systemically dehydated skin (not so easy to fix without client's participation). It LOOKS drastically different too. Fortunately, it's easy to fix. Start drinking! (H2O that is.)
The skin is a living organ that is always pushing things up and out: past sunburns, chicken pox, shingles etc. If you think of any of these unpleasant conditions you realize (thankfully) that they don't last forever. The skin is constantly repairing itself. It performs this work better in a hydrated environment. This is why they keep burn victims' wounds moist.
Case in point, I had a client who was complaining of migraines and lack of energy. You can imagine her poor skin reflected just that. It was dull, lackluster and way beyond dehydrated. I recommended (begged) that she drink more water and less coffee.
A week later I'm happy to report the massive positive change in her skin quality: smoother, clearer and more luminous. I was so proud! She was even carrying her water bottle with her. And no more migraines!