An underwatered workout is insufficient. You won't last long, you accomplish little, and it's hard to recover. Reason is, the human body requires water to lubricate the joints, digest food, transport nutrients, and flush waste. In addition, it helps carry oxygen and fuel to working muscles. Not to mention , in the form of sweat, it keep your body temperature in check when you're pushing through the last miles of your marathon. If you're not well hydrated, you have a drinking problem and need rehab. With that said, you need to drink, drink, and drink some more. If you wait until you're thirsty, it's to late. Try sipping liquids throughout the day.
And that's how to keep it moving! Until next time, stay safe.