I know that some people on this forum claim that they do drink water and have no problems, while others say that they tried drinking and it ruined the flush.
I used to follow the protocol exactly, and got very dehydrated. the last few flushes I had great success not drinking any water (after 2:00pm) on the day of the flush, then the next morning, after my first Epsom Salts , I started drinking water. This helped me immensly not to dehydrate and to recoup alot faster, and the flush was very successful.

Good luck