I've always drunk a lot of water every day, even before I had the pouch, around 2 to 3litres although I do a lot of exercise.
after takedown I was advised, at first to bring that down to a litre a day, as too much water can make you dehydrated as everything passes through too quick.
I was also told to make a rehydration drink and have a litre of that a day.
it helped a lot, after a month or so I didn't need it and I now drink the same amount of water I used to before I had the pouch. I never really get dehydrated.
fyi instructions for the home made drink are here:
http://www.surreyandsussex.nhs...-Electrolyte-Mix.pdf