Exercising increases our core temperature and as a result we sweat to regulate our body temperature. Through sweat we lose important electrolytes, and if this continues eventually we'll become dehydrated. Our perception of effort will increase, physical performance will dip, as will cognitive performance leading to; slower reaction times, increased fatigue and poor concentration. We certainly want to avoid this to make the most of the time spent during training and in a competitive setting!
To counter dehydration it's important to keep your fluids topped up throughout the day and additionally during exercise. A very rough guideline is 2 litres of fluids per day (although the actual volume will depend on your body weight), and an additional 500ml/hour of exercise (with a mix of electrolytes).
Remember:
...drinking only water and neglecting to replenish the electrolytes can cause hyponatremia, which can be life-threatening
...ensure you drink water mixed with electrolytes as your body needs both in order to avoid dehydration
...hydrate early and regularly. Sip, Sip. By the time you feel thirsty it's already too late!
...put in as much effort to properly hydrate during shorter-duration workouts as you would for a longer workout