It definitely helps to up your water intake. I kept getting up in the middle of the night last night with a lot of pain and I ended up drinking 72 oz. of water throughout the night and I feel great today.
Dry mouth is the #1 symptom that you're dehydrated. Granted I had to go #1 - 3 or 4 times but my mouth continued to be dry so I just kept drinking water.
One thing I wish the IBS books would better address is when do attacks most often occur after you've eaten something.