Here is an interesting rule of thumb used by elite athletes: for every 1% of under full hydration your body is, the resulting loss in physical performance is 2%! Put another way, for all the hours you exist and you are under hydrated by 1%, you will have to expend 2% more effort than you would if you had simply remained properly hydrated. 5% under equates to 10% more effort! More effort requires more fuel, more fuel requires more spend. Plus, more effort makes you tired sooner.
Being under hydrated also puts your immune system at risk. Water is a key component of antibody manufacture. Without them to fight off a bug, you get sick. If you get sick, that puts you behind on your goals and also puts you in a bad mental state.
Water is also cheap. The bottled water is so overrated. When we eat out, we always ask for tap water (unless we are outside the US).
Taking steps towards being fully hydrated are simple. For example, you could begin replacing your non-water beverage intake with water. If you drink 4 cans of soda, cut to 3 and replace that 4th one with the equivalent water. Another simple first step is to create a routine wherein each morning, the moment you wake up, you drink a glass of water. Viola! you've increased your water intake.
Over hydration is also a risk. Listen to your body. Do you need gallons of water? No. (unless you are racing accross a hot environment). There are plenty of formulas out there to help you gauge how much water you should be taking in. Find out how much you should be taking in and be sure you're doing it.