Although the chances of your stool turning red from consuming watermelon are extremely low because of the sheer amount you would have to eat, it is in theory possible.

Usually the variation of stool color has more to do with the content of the food itself and not so much the color. Most of the time, dark stool such as the color red, is due to consuming foods such as blueberries as well as iron supplements.

Even the stomach medication Pepto-Bismol has been known to turn stool red; however there is very little documentation of watermelon doing this in this who consume a generous amount of it at one time.