If the dive is in the afternoon, keep the portions small and choose high-carbohydrate foods and sports drinks or water for lunch. You should be drinking water up to one-half hour before the race. About 10 or 15 minutes before the have a water bottle handy and take in two-four ounces of water. Water consumed this close to the dive start will help counteract the obligatory diuresis of your initial water immersion. Depending on the number of dives, water is usually best.
Most dive boats have a supply of cold water available with plenty of cups.
I've not seen any divers using any of the 'sports gels' that runners use. Avoid alcohol as long as possible the few hours after a dive as it will dehydrate you as well as slow the restoration process of your liver and exercising muscles. While following these tips may not allow you to dive any deeper or use less air, it can make a big difference how you feel during and after the dives. It is just possible that you might ward off getting bent and ruining a good vacation or even having permanent neurological damage.