Hydrate and fuel yourself! I generally just do water, but on long hot days I use additives. Keep the fluids going, and learn how to go to the bathroom in your boat. Paul Petzold said the lunch was the meal that starts right after breakfast and ends just before lunch. Feed yourself. I am not a fan of most 'power food' but it can work in a pinch. Nuts. Cheese. Pepperoni. Carrots. Peanut butter. And keep it where you can get to it, like a drybag in your cockpit.