Remember that you can take as much you want, but you have to carry it. Personal hygiene and sanitation are a must when in the field as most experienced troops will tell you. Wear loose fitting uniforms for better ventilation and when worn properly for protection to exposed skin. Thirst is not a good indicator of dehydration. If your urine is dark yellow you are not drinking enough water.
Refill canteens with treated water at every opportunity.
Wash your hands thoroughly before preparing food, before and after eating and using the latrine. A sure way to get diarrhea is to use dirty mess ware. Wash you kit with hot water or a disinfectant solution when possible. Brush your teeth twice daily, and when possible take a wash cloth daily and wash your genital area, armpits and feet. Personal hygiene methods, by their very nature, are designed to protect you.