This is perfectly normal!
Your pharmacist or doctor should have told you to expect it.
In the first week, as your body gets used to the new diet (for me it happened on day 3) you may go through a period where you get loose stool or feel like you have diarrhoea. This is temporary and will pass, most likely by day 7.
It is not because you are drinking too much water.
At the risk of TMI (too much information), let me quote from my weight loss diary for a sec (because I'm brutally honest in it).
"Now, the next bit is a bit disgusting, so I'm giving you fair warning... But hey, anyone on this program should know what to expect, including the bad...
While sitting and reading, I felt what at first, appeared to be a fart. And it wasn't. Eeeeewwww....
I went straight to the bathroom, and cleaned myself off, and took a very watery dump. I guess that is also to be expected, on a mostly fluid diet with no solids. I will have to be aware of this possibility especially in the office. Perhaps keep a spare pair of trousers/underpants in my car, in case of accidents... I might also want to try some of that Clear Fibre which they recommend as a supplement for this diet.. To keep the colon well maintained. Apologies for the grossness, but if I'm going to chronicle the good and the bad on this program, well, it had to be done."
So you see, I also thought it was something that might continue for a while, because my pharmacist was also a useless tosser.
In a nut shell, don't worry about it.