If you ever feel thirsty during exercise then it is extremely likely your body is entering the first stages of dehydration. This is why it is so important to get into the habit with drinking water during long duration exercise – before you feel thirsty. And it is important to get the amount right – more is not necessarily better (if you have not heard of hyponatremia then read about it here).
Exactly how much you drink depends on what type of activity you are doing, for how long, at what intensity, and in what atmospheric conditions (it is impossible to make generic prescriptions on this one). An easy, but somewhat crude way to measure what you need is to see how much you are sweating (and therefore how much you need to replace). Weigh yourself naked and dry before and after exercise. The difference is mainly what you lost as sweat, and hence the approximate amount of fluid you need to ingest.
Carbohydrates are very important to consider, however the short term side effect of not ingesting enough carbs during exercise is probably just going to be exhaustion. Not nearly as severe as the short term side effects of dehydration.
