I remember reading a poster back in school that said "If you're feeling thirsty, you're already dehydrated." We know that hydration is key to survival. You can survive longer without food than you can without water (picturing myself drinking from coconuts on a desert island, mmmm), but it's a lot easier to forget to hydrate than it is to forget to eat.
Hydrating is also key to good skin and is often confused with moisturising! When your skin is dehydrated it means that your skin lacks water; it will feel less firm, less elastic and not as soft or smooth. When your skin is dry it means that your skin lacks oil, that's where moisturiser steps in.