The common answer to this question is that you should drink water when you are hydrated. It is also common to hear that you should avoid drinks such as coffee, tea, alcohol, milk and juice as they will not help to hydrate you.
However this is a common misconception as virtually any fluid will help to hydrate you, though you should steer clear of seawater (not that you would want to drink that anyway!).
There is no scientific basis for this curious idea that fluids other than water cause dehydration and so you should avoid them if you are already dehydrated. As a diuretic (something that makes you pass water) the majority of fluids will help to rehydrate you, although caffeine does cause a slight loss of water, it's only a fraction of what you are adding by drinking the coffee and so even caffeine will is fine. Tea, coffee, milk and squash are all equally as good at replacing fluids in children as well.
Ron Maughan, Professor of Human Physiology at the University of Aberdeen Medical School, has looked at the effects of alcohol which is considered to be another diuretic, found that in moderation, contrary to popular belief, it too has little impact on the average person's state of fluid balance.
His results, which have been published in the Journal of Applied Physiology, showed that alcoholic drinks with an alcoholic content of less than 4 per cent, common in light beers and lagers can actually be used to stave off dehydration.
However, seawater is an emetic, which means that if you drink it, you will throw up. If you do manage to keep any of it down, it will cause all of the water in your body's cells to move towards the highly concentrated salty fluid, by osmosis, in an attempt to dilute it.
This will leave your cells severely dehydrated and in severe cases can lead to spasms, the breakdown of brain functions, and liver and kidney failure (though again, you're unlikely to reach for a glass of seawater when you are dehydrated after exercise. You may be tempted if you are stranded at sea with no fresh water, however this will do you more harm than good.)
In summary, contrary to what you have probably been told by parents, coaches and other people after exercise or whenever you are dehydrated, you do not have to drink water to rehydrate yourself. Milk, juice, coffee, tea can all be used to replace loss fluids and even alcohol can held prevent dehydration. Although water is obviously a very good source of rehydration, many people do not particularly like water but think this is the only way that they can rehydrate themselves. Hopefully this article has shown that this is not the case and pretty much any fluid, as long as it is a diuretic can be used for rehydration. Just make sure you steer clear of seawater!