I keep reading statements like this (this one from a BBC news article, talking about alcohol and mangoes), and get suspicious that there is a misunderstanding somewhere:
It acts as a powerful diuretic, causing you to urinate more, meaning you become dehydrated.
It seems to me that it would be one of these:
It acts as a powerful diuretic, causing you to urinate more, meaning your bladder gets emptied more often, but otherwise not altering the water levels in your body. Diuretics are mainly a problem just before bed and on long car journeys.
It acts as a powerful diuretic, meaning more of your body's water gets redirected to the bladder, even more than the water in the diuretic drink, and so you end up with a net loss of water in the body.
I thought "diuretic" had the former meaning (you want to empty your bladder more often) but I'd love to have a proper medical reference, or see some solid scientific research.