Cut watermelon into large chunks (you should have 6 cups). Puree half the watermelon, lemon juice, and salt in a blender until combined. Cut ribs and remove seeds from chiles, and cut into 1-inch pieces. Add to blender with remaining watermelon; pulse until chunky, 2 or 3 times. Refrigerate until cold, about 1 hour. Stir well, and garnish with thinly sliced jalapeno if desired before serving.