Hoplon said:
Alcohol and Caffeine are diuretic though, they make you pee, so the net balance is loss hence why you need to drink water.
Well, alcohol, yes, but I doubt you (or at least hope you don't!) drink that permanently.
Caffeine, up to a point.
For example, tea is better than water for rehydration http://news.bbc.co.uk/1/hi/health/5281046.stm
Admittedly tea doesn't contain as much caffeine as coffee but it contains a fair bit, around half the amount of instant and a third of filter, so containing caffeine doesn't immediately mean you have drink an equal amount of water.
There is plenty of stuff about debunking this odd obsession with water drinking. Annoyingly I can't find the Oxford University study which obviously has much more weight than some random 'sites. Sorry!
http://www.npr.org/2008/04/03/89323934/five-myths-about-drinking-water
http://www.sfgate.com/health/article/No-need-to-drink-8-glasses-of-water-a-day-3650324.php
Bottom line is there is no scientific reason to drink water unless you feel thirsty and even then tea, milk or just eating an orange will do just as well.
Edit - sorry for double post.