60% of your body is water and has many roles including temperature regulation. Losing 2% of your body weight as water greatly affects your performance both physically and mentally. Dehydration causes muscle function to decrease, energy levels to drop and fatigue to increase. All of which you want to avoid if you are trying to make the most out of your training. A 5% loss can be fatal.
A rough guideline is 2 litres per day, but this varies greatly from individual to individual and also dependent on your activity levels and the humidity. Next time you go to the toilet, have a look at the colour of your urine as a guideline of your hydration levels. Generally, the lighter the colour the more hydrated you are.
So grab a bottle (of water!) and get training.