Hot midsummers day with the temp in the 80's (once the sun came up) and a long 14 hour ride that was going to make you sweat. I had one pee after about 3 hours and that was the only one of the day. The 1 litre per hour of water was exceeded by 25% and I lost no weight on the day- but that was probably down to the amount of food consumed to replace what we were burning on the ride and The BIG bucket of Greasy KFC after the ride.

I do not know about over drinking during exercise- But I have felt the effects of de-hydration a few times. That has finished me for a couple of hours each time.