I believe that sticking to the program also keeps the right PH factor from the citrus (oj)... VErY IMPORTANT.
Although some relief may be obtained,and it may work for others, I am not overly confident that Watermelon juice or any alternative would be worth throwing the system / the ph out of wack. Risky in my opinion ,especially after 21+ days.
I will concede that a short 10 or less cleanse could be able to handle alternatives and substitutions without as much risk.
Yet,it could make things worse, especially if you are at a week point...
Please take it easy and start planning that soup! Yum...
and lots lots of H2o