01 - Combine graham cracker crumbs and sugar in a bowl. Pour in melted butter and stir until fully incorporated and mixture resembles wet sand. Press firmly into the bottom and up the sides of a 9-inch pie dish. Refrigerate for at least 30 minutes to set the crust structure.
02 - Blend seedless watermelon in a blender until completely smooth. For a silkier texture, strain through a fine-mesh sieve to remove excess pulp. Set aside.
03 - Sprinkle gelatin powder over cold water in a small saucepan. Let stand undisturbed for 5 minutes to allow the gelatin to absorb the water and bloom. Gently heat over low heat, stirring constantly until gelatin dissolves completely. Do not let it boil.
04 - Combine watermelon puree, sugar, and lemon juice in a large bowl. Gradually stir in the dissolved gelatin mixture. Let cool to room temperature, approximately 10-15 minutes, stirring occasionally to prevent a skin from forming.
05 - Gently fold whipped topping or whipped cream into the watermelon mixture using a rubber spatula. Use a light folding motion to maintain airiness. Continue folding until no white streaks remain and mixture is uniformly pink and smooth.
06 - Pour filling into the chilled crust. Smooth the top with an offset spatula. Refrigerate uncovered for at least 4 hours, or overnight for best results. The pie is properly set when the center no longer jiggles when shaken.
07 - Top with additional whipped cream if desired. Arrange fresh watermelon balls or slices and mint leaves decoratively on the surface. Serve chilled. Slice with a sharp knife dipped in warm water for clean cuts.