Gotta jump in here...
When diving, as with any exercise, it is vital to drink plenty of fluids and stay well hydrated. Diving and other water-based activities are somewhat misleading in the amount of fluids we lose to the environment. When diving, you will lose large amounts of fluids to the very dry air. You see, you must have moist (near 100% humidity) air to move the gasses across the membranes in your lungs. So what the air supply does not give (moisture) the body adds.
Now, on the subject of rehydration...
Water is the best -- little doubt. However, most people cannot drink large quantities -- they don't find it palatable. Several suggestions: use the sports drinks, but dilute them by 1/2 to 1/3. I drink Gatorade at 1/3 strength, and I find myself drinking fluids more aggressively than without flavoring. Additionally, the sugar in the Gatorade helps fuel your efforts. If you don't want Gatorade, try adding a small amount of pure lemon juice to the water. Studies have showed that humans strongly prefer fluids with a citrus taste. Do not neglect to eat while diving, humans are accustomed to eating and drinking -- and water intake is much higher with a meal than with fluids alone.
Water intoxication or symptomatic dilutional hyponatremia is real, I've treated it. It is caused when your intake of water exceeds your kidneys' ability to clear the free water, thus driving down the sodium levels in your blood. Your gut is much more effecient at absorbing fluids than your kidney are in clearing it. Current recommendations are not to exceed about 1.5 quarts of water per hour and do not exceed 12 quarts per day. (That's three gallons of water!) Watch for nausea and vomiting. If you feel nasty and vomit once, it is probably heat exhaustion -- rest, cool off in the shade and drink some fluids. If the vomiting persists (more than twice in two hours, could be hyponatremia -- get it checked out.) In the most severe cases, victims will have seizures and death. However, if you drink water in an intelligent fashion as outlined above, you will most likely stay out of trouble. This is pretty common in the Army where rehydration is heavily emphasized and sometimes the soldiers get a little over vigorous. One patient I treated at 1:00 pm for this had had 2.5 gallons of water in six hours.
As always, I provide this information for your education only and this does not constitute a physician-patient relationship. If you have any questions, I urge you to check with your personal physician.