Here are some useful tips about water and why you need it.
1) Why you need water | Your body is around two thirds water, so you're more of a puddle than anything else. Your body can go a few weeks without food, but you're going to run in to serious trouble if you don't have any water for a few days. Water has many jobs within the human body - It protects the brain and spine, removes waste products and toxins, helps to regulate body temperature, helps with digestion, and more. You lose water through sweat and excretion. Regular exercise and hot weather can easily cause dehydration.
2) How to tell if you're dehydrated | The most obvious clue is that you're going to feel thirsty, very thirsty. Feeling exhausted after a workout/exercise/competition could mean dehydration if you haven't taken on too many fluids. Feeling light headed could be another indicator of dehydration. The colour of your urine will tell you about your hydration levels - a light colour means you're fine, but the darker it is, the more dehydrated you are. You know how much you drink during the day, so if you know you haven't drunk too much water (or have drunk a lot of alcohol, on holiday for example) then you'll be dehydrated.
3) How dehydration affects performance | Dehydration affects performance more than many other factors. Even the smallest drop in hydration levels can negatively impact your performance output. A study at a Californian University showed that a 3% drop in hydration levels (which sounds like nothing) can significantly increase your chances of exhaustion, and even heat stroke. A lack of water can cause you to lose mental focus, concentration, strength and recovery. Your brain is like an engine - the more demand, the hotter it can get, so you need to cool it down. Your body too.
4) How much water do you need | Most medical websites advise drinking around 7-8 average sized glasses of water per day. The main problem with this is that it's a generic guide, it doesn't take into account heat, exercise level, gym, competition etc. Coming from a pure fitness point of view, I would recommend an intake of around 3-4 litres for men and 2-3 litres for women. If the weather is excessively hot or very intense exercise, you may need a little more, possibly even some electrolytes.