I must go through about 10 cups of tea & coffee a day (mostly tea). I cannot remember the last time I drank any water, I think it was during particularly strenuous exercise. I have absolutely no signs of dehydration and I feel fine. My body has been telling me my whole life what it wants to eat & drink and I've been ignoring it. Only in the past 2 months when I've actually started paying attention to it have I felt better and started losing weight.
I think it's no coincidence that I recall this frenzy of absolutely HAVING to drink 8 glasses of water a day (nothing else, just water, oh and none of your dirty tap water, it must be mineral water) popping up in the early 90s, at the same time as the bottled water industry became so huge.
To me, it's nothing but folklore: marketing that has crept insidiously into medical advice.
Basically, I think this makes more sense: drink fluids, you need them. What you need will depend on your height / weight / composition / diet / exercise / hormones / environment / genetics. Your body will usually keep you right - that's why it has hormones to send signals to your brain to articulate your need for... whatever it is. If you like water, drink it. If you like tea, drink it (this doesn't really apply to stuff like coke that's full of sugar
).