01 - Combine the graham cracker crumbs and sugar in a bowl. Stir in the melted butter until fully incorporated and the mixture resembles wet sand. Press the crumb mixture firmly into the bottom and up the sides of a 9-inch pie dish, creating an even layer. Refrigerate for at least 30 minutes to set.
02 - Add the seedless watermelon cubes to a blender and process until completely smooth. For a silkier texture, strain the puree through a fine-mesh sieve to remove excess pulp, pressing gently with a spoon to extract all the liquid.
03 - Sprinkle the gelatin powder evenly over the 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 completely dissolves. Remove from heat immediately—do not let it boil.
04 - In a large mixing bowl, whisk together the watermelon puree, sugar, and lemon juice until the sugar dissolves. Gradually pour in the dissolved gelatin mixture while whisking continuously to ensure even distribution. Let the mixture 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 folding motion to incorporate without deflating the mixture, stopping when fully combined and smooth with no visible streaks.
06 - Pour the watermelon filling into the chilled crust, smoothing the top with an offset spatula or the back of a spoon. Refrigerate for at least 4 hours, or until the filling is completely set and no longer jiggles when the dish is gently shaken.
07 - Before serving, pipe or spread additional whipped cream around the edge of the pie. Arrange fresh watermelon balls and mint leaves on top for garnish. Slice with a sharp knife dipped in warm water for clean cuts.