thatbags - I don't really remember, as this was about 12 years ago. I think with tea and coffee, the tea, coffee, milk, sugar (if taken) have to be processed by the body but with plain water, there's nothing really to be processed. Galen would be better versed in this than I would be.
I'd been given an abdominal scan and the flabby kidneys showed up on that (they weren't the reason for the scan). The consultant asked if I drank a lot of water and when I told him that I supposed I did, he asked how much - as I would drink the same amount of water that most people would drink combined tea/coffee/soft drinks/alcohol - I suppose it was just too much. I did notice, once I started drinking tea and coffee that I didn't run to the loo as often so water definitely goes straight through you - maybe the kidneys were just overworked? I did cut down on the water (only about 5 cups a day) and within a few months, the kidneys were fine again.
I think it's like everything else, moderation being the key.