No, don't eat watermelon. The closest thing you're allowed to solid food is jell-o (no red or purple flavors, I stuck with pina colada as that was really light yellow color - anything reddish might appear to be blood in the digestive tract so no red jell-o nor red drinks). You should be able to have clear broth, water of course, clear sodas, ginger ale, clear sports drinks, etc. I think light-colored teas were also allowed when I did prep.