Water is the most important thing when outdoors, and I've found myself not drinking enough, if you have a water source you should stick close to it, but as others have posted it's funny how the mind works, it just over this next hill and I'll be out of here, this is the type of thinking that will end up causing you serious problems.

When I have the boys out on trips we make regular water stops, they are pretty good at calling a water break long before I would think I'm ready for it, when you get them use to this type of hiking you hope that thought becomes second nature to them

depending on weather condtions I put thing into this order

shelter (I live in a cold region)
water
food