I've done intermittent fasting with good results so far. Though I don't do it for pouch function but it definitely improves my functioning.
I use it to cut back fat gained while bulking up. Won't suggest you to start straightaway with a 20-4 fast instead go for a lighter 14-10 or 16-8 fast. Your body may need some time to adjust to the fasting time.
As for the water part I drink upto a litre of water everytime I have to and thats 4-5 times a day minimum. I have to pee that much but thats fine I just want to stay dehydrated.