While in general it's a good idea to drink lots of water during a cleanse, there are at least 2 possible areas of concern: 1) If you have high blood pressure, too much water in your system can raise it even higher, and 2) more commonly, excess water can dilute electrolyte levels in the blood, primarily Potassium and Sodium to the extent where they are below recommended levels.
So at the very least you should assure that you are getting enough of essential minerals, either in your diet or from supplements. And if you tend toward high blood pressure, you should consult a naturopath or other health care provider for advice and guidance.