The Master-Cleanse is actually a great kidney cleanse. So no worries there.
As to post-MC weight gain, it's probably normal to gain about 5 pounds or less of it back just from refilling the colon. However, anything beyond that is not necessarily par for the course (unless you're underweight to begin with).
Excess weight gain is a direct result of eating more calories than you can burn, and eating the wrong kinds of foods, causing your body to become toxic once again. As with any kind of eating plan you have to be smart and stick to it to keep the weight off. So the best thing to do would be to make a plan now (while fasting) as to how you'll eat when you're done.
As to only wanting water ... listen to your body. If it needs a day or so of water only, then do that. No harm done. However, I personally don't recommend Water Fasting for more than 3 days at a time without a doctor's supervision. It can be dangerous. God bless and hope all goes well!