 Not drinking enough water can result in dehydration, though says the first sign of dehydration is thirst. The other things that can kind of progress from there is you'll go to the bathroom less often. When you pee, it will be darker in color, and so that's another hint that you haven't been drinking enough water if your pee is really dark. And then as dehydration can progress, you may experience other things like dizziness, lightheadedness.