Let's make it
Preheat oven to 350°. Line 8-inch square baking dish with aluminum foil so that foil extends about 2 inches over sides of dish; spray with cooking spray.
In medium bowl, combine graham cracker crumbs and butter; firmly press into bottom of prepared dish and bake 15 minutes or until browned. Cool crust on wire rack.
In blender, purée watermelon on high until smooth. Makes about 2 1/2 cups.
In small bowl, whisk gelatin and puréed watermelon; let stand 5 minutes.
In medium saucepan, heat vanilla bean, lime juice, sugar, coconut milk and gelatin mixture to a simmer over medium heat; remove from heat, pour over crust and refrigerate 2 hours or until firm.
Using overhanging sides of foil, transfer bar to cutting board; remove foil and cut into 16 squares. Serve bars topped with whipped topping garnished with lime zest, if desired.
Approximate nutrition values per serving: 112 Calories, 5g Fat, 3g Saturated Fat, 8mg Cholesterol, 44mg Sodium, 18g Carbohydrates, 1g Fiber, 11g Sugars, 8g Added Sugars, 1g Protein