When training for a long period of time (usually more than 90 minutes), water might not be enough to replenish you. What about those sugar-loaded sports drinks? Are they good for you? The intention behind them is to give you the electrolytes and minerals (salt) you are losing during your workout.
I use coconut water, which contains natural electrolytes. Coconut water is a natural thirst-quencher that will help you rehydrate without all the added sugar of some sports drinks. It also contains potassium.
Be careful, not all the brands of coconut water are the real deal. Read your labels. Some of them might be loaded with sugar.