I have learned a lot about this as I have studied exercise science...and I'm agreeing with most of what I have read on this thread. It does take drinking an awful lot of water before you need to worry about having drank too much, it is MUCH more likely to not drink enough. Also, drinking small amounts of water often is more effective in maintaining balance under severe heat or exercise conditions than is drinking very large quantities at longer intervals. one rule of thumb is 600mL-1L per hour of rigorous exercise. In hot weather espcecially, thirst is not a good guide to dehydration, and a little extra water will do no harm. If you wait until you're thirsty, you are already dehydrated. The best thing to be drinking is usually water, as most athletes do not need the amount of electrolyte replacement as sports drinks provide. In fact, sports drinks, rather than adequately hydrating an athlete, can hinder water absorption because of their high sugar content. Too much glucose can draw water from blood (where you need it) into intestinal tract to help in digestion. Drinking gatorade is not bad, but you should drinking much more water than gatorade. Gatorade can be more beneficial, toward the end of a long bout of exercise, when you have depleted some of your glycogen stores. And stay away completely from sodas they've got caffeine, carbonation, and twice as much sugar as sports drinks. Also, "All sport" (like gatorade) is carbonated, and should be avoided.
As for nutrition, this is most important in the days leading up to competition, not the day of. A Balanced diet will provide all the necessary vitamins, minerals, electrolytes, etc you will need, and won't require supplementation...but go ahead and add a little extra salt to your food, and eat a bananna if you want. The salt is more important than potassium from banannas, potassium loss is usually negligable. This type of electrolyte replacement is especially useful for unfit people, or for when exercise will be resumed again within a few hours of stopping. What you eat is most important about 48 hours to competition. Serious Carbo loading can be useful, but only in LONG bouts of exercise like the second half of a marathon, or a 1000 mile race.
Okay, I think I've droned on long enough...sorry.
Jaron