The human body is composed of 60-70% water and it is crucial to replenish water levels daily through drinking. A person can survive a month without food but only 3 days without water. Water is lost through daily activities like sweating, breathing, and urinating. The amount of water needed varies based on weight, climate, and other fluids consumed, but as a general guideline, drink half your body weight in ounces per day. It is important to re-calculate water needs as weight changes and drink water frequently throughout the day to avoid dehydration.