A study referred to in a BBC NEWS article found that

Drinking three or more cups of tea a day is as good for you as drinking plenty of water and may even have extra health benefits, say researchers.
The work in the European Journal of Clinical Nutrition dispels the common belief that tea dehydrates. Tea not only rehydrates as well as water does, but it can also protect against heart disease and some cancers, UK nutritionists found.

Cheers!