Back in highschool I used to do weight-lifting. As in the olympic event you see on television where big guys and gals are pulling great barbells of weight off the ground and over thier heads. I was pretty good at it, but always found it hard to put on weight, and could never really progress in the sport. In any case, one thing that my coach always used to push me to do was to come to training well hydrated. Drink plenty before, during and after. He told me to weigh myself before and after exercise to calculate how much I need to drink while training. He told me to drink at least 2 cups of water a good hour or so before exercising. I always noticed that when I was properly hydrated, I always felt more energetic and stronger.
While your body is working hard, you are losing water to sweating and breathing and soon enough you begin to feel thirsty. You need to sweat to keep your body cool and you need to breathe to get enough oxygen. But you loose water to both. Loosing too much water can dehydrate you (even a loss of 2 percent water is considered dehydration) and affect your performance negatively. Drinking a small bit of water every 15mins or so during a workout is a good way to keep your fluids up. Additionally, if you are exercising for a good hour or so, you will properly need to replace lost electrolytes (salts). A sports drink is okay, but fruit juice is just as good.
Keep hydrated during exercise and you keep your body performing at its best.