Heat 1 cup each sugar and water in a saucepan over medium heat until the sugar is dissolved; cool. Puree 6 cups chopped seedless watermelon in a blender. Pour 3 cups of the watermelon puree into a pitcher; add 3 cups vodka, 1 cup fresh lime juice (from about 10 limes) and 1 cup of the simple syrup. Serve over ice and garnish with mint. Serves 8 to 10.