For watermelon shrub syrup: In a large bowl, combine the watermelon and sugar and stir well to coat. Cover the bowl and refrigerate overnight. The next day, mix together the vinegars in a small bowl and stir. Add the vinegars to the watermelon, stir and place back into the refrigerator. On the third day, fine strain the solids out and store the shrub syrup in the refrigerator until ready to use.