I have also heard that if you drink too much water it can give you like poisoning??? But I think we are talking extreme amounts of water here. Not sure where I heard it either.
for WW you only need to have 1.5litres a day of any fluid .. it states in the book that obviously water is healthy for you, however it doesn't matter if your intake is made up of tea/coffee/diet drinks... as long as it's 1.5 litres in total for the day that's what you should stick too
for me it all depends on how i feel that day, sometimes i drink water all day, others it's diet drinks / squash etc... somedays it's only a pint others it's nearly 3 litres.. lol