The body is made up mostly of water, and as you exercise you lose that water through sweating, which is the body trying to cool itself. If you don’t replace that water, it causes the water in the muscle to be used to cool the body, so the muscles because less loose and tighten up, cramping. If you continue down this path, you end up with your nervous system being affected, since water is essential for the electrical pulses that are used for the body to communicate with the different parts. That means you will get fuzzy headed, dizzy, lose motor skills.
To prevent this, is quite simple. Drink! For short workouts, under an hour you can get away with just drinking water. But if you go over that hour or so mark, then you also need to start replacing the electrolytes (salts) that you sweat out as well which are also important to keeping the body moving. Salt water conducts electricity better than fresh water (remember those elementary school science projects?) So you should start drinking sports drinks or taking electrolyte tablets to help replace them.
How much should you be drinking? Average person can drink about 24 ounces and be okay, that’s a large bike bottle worth. Personally I like to set my watch to beep at me every 10 minutes and I take a decent mouthful of water or sports drink. So I’m drinking roughly four ounces six times an hour. When it’s hotter and humid (welcome to Houston!) I drink quite a bit more, and can quite easily go through 50 ounces in an hour.
You do have to watch out, as you can drink too much and get hyponatremia, or water poisoning. This causes too much water in the system and not enough electrolytes causing an imbalance and your electrons can’t move around as freely (back to that whole fresh water versus salt water thing.) It can be quite dangerous two fold, your body will start to not function correctly, with the same symptoms of dehydration which is where the two fold part comes in, people will think you are dehydrated and get you to drink more, just making it worse.
Everyone is different, and so you need to play around with things to see how they work best for you. Start off with a water bottle, and try to drink from it through out an hour. See how you feel. If you feel bloated, it’s too much, if you feel thirsty, it’s not enough. Nutrition isn’t just science, it’s an art that requires playing around with to make it work best for you. Even when you think you have the best solution, continue to play around a bit, maybe you will find something even better.