01 - Combine the graham cracker crumbs and sugar in a mixing bowl. Pour in the melted butter and stir until fully incorporated and the mixture resembles wet sand. Press the crumb mixture firmly and evenly into the bottom and up the sides of a 9-inch pie dish. Refrigerate for at least 30 minutes to set.
02 - Add the seedless watermelon to a blender and process until completely smooth. For a silkier texture, strain the puree through a fine-mesh sieve to remove excess pulp. Set aside.
03 - Sprinkle the gelatin powder over cold water in a small saucepan. Let stand for 5 minutes to allow the gelatin to bloom and absorb the liquid. Place the saucepan over low heat, stirring constantly until the gelatin dissolves completely. Do not let it boil.
04 - In a large bowl, whisk together the watermelon puree, sugar, and lemon juice. Gradually stir in the dissolved gelatin mixture until well incorporated. Allow the mixture to cool to room temperature, about 10-15 minutes.
05 - Gently fold the whipped topping or whipped cream into the watermelon mixture using a spatula. Use a light folding motion to maintain the airy texture until fully combined and no white streaks remain.
06 - Pour the filling into the chilled pie crust. Smooth the top with a spatula or offset spatula. Refrigerate for at least 4 hours, or until completely set and firm when gently touched.
07 - Before serving, top with additional whipped cream if desired. Arrange fresh watermelon balls or slices and mint leaves on top for garnish. Slice and serve chilled.