Here's your solutions in 6 easy steps. Assuming of course that you are diving a safe conservative dive profile.
Eat a good breakfast. Stay hydrated. Eat continuously throughout your dive day/between dives. Stay hydrated. Get Nitrox Certified. Stay Hydrated. =)
As relaxing as diving is, it still takes a lot out of your body. Especially continuous diving. You need calories to burn for energy and warmth even in warm water. Proper hydration keeps your body functioning optimally. Some of old school divers who dove canisters filled with whisky and pumped by their garage compressor and before there were real certifications will say that nitrox doesn't do anything for you and is a waste of money. I fully disagree. Having led many dive trips, I can say for certain, because I was actually curious myself, that the groups that dove nitrox tended to have a lot more energy, dove more, and would be up for more post dive shenanigans than the groups that did not dive nitrox. It's a relatively inexpensive class. If you could do it there, I believe it will enhance your experience.
Hope that helps!