I think the 8 cups of water rule is ridiculous! A 350 lb man and a 90 lb woman have very different needs in terms of hydration. I think plenty of water is necessary for everyone but the extract amount needed varies wildly from person to person. Environment is another good point you made. When I lived in Florida it felt like you needed 8 gallons of water a day in the summer! On the flip side living in Maryland in the wintertime allows me to run on much less water. Dehydration is dangers but so is being over-hydrated! Studies have shown dry fasting (abstaining from food AND water for extended periods of time) can actually be beneficial to one's health. So I think the question is not how much water should everyone drink in a day but rather how much should each individual drink to meet their needs.