This week (on Wednesday) The Guardian posted an article on the food marketing myths we've "swallowed whole". It's entertaining as well as informative, and is well worth reading. I'll add the link below.
One of the myths was that we need to drink eight glasses of water a day, which they say is very good if you enjoy urinating. While we do need 2.5 litres of water a day (the answer to your question) we get most of that from the normal food we eat.
Here's a link to Take It With A Pinch Of Salt.