Chris Kresser on the 8 8oz glasses of water a day recommendation:
The best advice I can give when it comes to water consumption is let your thirst be your guide, and adjust your fluid intake depending on your lifestyle and environment. If you are relatively sedentary, live in a cold climate, don’t sweat much, or eat water-rich foods, you probably don’t need to be drinking eight cups of water a day. There’s no evidence to support those recommendations, and forcing down excess water throughout the day is not only unnecessary, but can cause damage if done too frequently. For most people, the body is its own best guide.
I agree. Evolutionary science is not my strong suit, but it seems unlikely to me that historically our ancestors were chugging this much water on a regular basis. And if they didn’t, then it seems unlikely it’s a necessity for us, unless, as Chris says, your specific situation calls for it.