There a few things as important for your skin as water. Whenever your body becomes dehydrated it draws on its water reservoirs, one of which being your skin, and sucks out all the moisture it can find. 
The resulting dry skin, not only looks unattractive but it also speeds up the appearance of wrinkles and other age-related skin deterioration.
Although it’s crucial to drink plenty water to keep your skin hydrated from the inside, it’s also vital to give it a helping hand on the outside as well.
A good moisturiser not only adds essential moisture to your skin but it also acts as a barrier to prevent moisture from escaping. An occasional face mask can also give you skin a much needed boost, particularly if you apply it just before you have a hot bath. And finally, when you wash your face, make sure you rinse thoroughly.
You must fully remove any soap or cleansing products from your skin to avoid blocked pores and skin irritation which can effect your skin’s natural moisture balance.
Source: http://www.womenrepublic.co.uk/beauty/water/