Your body is made up of two-thirds water and when you exercise much is lost through sweating, breathing and talking. In warm weather you lose even more! Losing fluid can cause dehydration which leads to premature fatigue and reduced performance. When you sweat you lose both fluid and body salts, which further impacts on your ability to play as well as you can.
The recommendation for daily fluid consumption are 2 litres or 8 glasses. This can come from a combination of water, milk, juices, tea etc. For sports people, it is important that you meet, if not exceed these recommendations. Thirst is a poor indicator of dehydration. By this stage your performance could already be impaired.
See chart below for more info.