Well I definitely have that trouble on this VLCD. Tried everything, and I've settled on Dulcolax Pico Perles that are effective and milder than the original Dulcolax. I use them a couple of times a week now when I need to and don't have the problems I had before.
I am discovering different reactions: if I have the liquid packs(soups and shakes), I get the runs. If I have only bars and porridge/spag bol, then I don't "go". So I ensure that I mix 'em up. I also make muffins with my choc shake or cookies with the porridge and that helps to to regulate me.
And like you say: keep up with the water.
I've always had a cast iron stomach unfortunately (or fortunately depending on how you look at it!) and I haven't yet found anything that has that effect on me unless I ate something that was completely 'off'.