Stir the sugar and lime juice together until the sugar dissolves. Put the watermelon into a blender and puree. Add the mint and pulse, so that it is coarsely chopped but not pureed - you want to be able to see pieces of mint. Pour the watermelon puree into a pitcher, and stir in the sugar-lime juice, rum, and sparkling water. Fill six glasses with ice, and pour the mojitos into the glasses. Garnish with mint leaves.