Please review these  few exerpts from an article by Lawrence at Finesoccer

Because this is a subject I have NO expertise at all, I will stick with the very basics.

In many areas here and in the U.S. (and other parts of the world) the weather has become quite hot and coaches and players have to be really aware of the dangers of dehydration.

Players and coaches need to drink a lot of water starting the evening before an event and continue throughout the day of the event.  It's important to realize that a player can't wait till they are thirsty before they drink.  They must stay hydrated at all times.

Some of the consequences of dehydration include:

Heat cramps, which can result in brief but painful muscle cramps as a result of insufficient liquid intake.

Heat exhaustion which can result in difficulty breathing, headache, feeling hot on head and neck, dizziness, heat cramps, chills, nausea, irritability, vomiting, extreme weakness or fatigue

Heat Stroke which can result in rapid and shallow breathing, rapid heartbeat, unusually high or low blood pressure, lack of sweating, mental confusion and disorientation, unconsciousness, physical collapse

When you start dealing with issues such as heat exhaustion and heat stroke you are dealing with issues that can result in long term disability or even death.  Too many coaches think they can go "just a little bit longer" or that it's not important for the players to stay hydrated because "when I was their age I could play without water for hours".  It's this type of attitude that results in short and long term damage to players.

Drinking a lot of water at one time isn't as effective as drinking a reasonable amount of water over a longer period of time.  If you drink a lot of water at one point, you are most likely to rid yourself of this through urination so drinking normal amounts in advance of the training will be in your best interests.

There are numerous other sports drinks a person can drink besides water and I will leave that for someone who knows a lot more than me but I do know water works.