01 - Combine graham cracker crumbs and sugar in a mixing bowl. Pour in melted butter and stir until fully incorporated and mixture resembles wet sand. Press crumb mixture firmly into the bottom and up the sides of a 9-inch pie dish. Refrigerate for at least 30 minutes until set.
02 - Add seedless watermelon to blender and process 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 for 5 minutes to allow gelatin to absorb water. Gently heat over low heat, stirring constantly until gelatin dissolves completely. Remove from heat.
04 - In a large bowl, whisk together watermelon puree, sugar, and lemon juice. Gradually stir in dissolved gelatin mixture until well incorporated. Let mixture cool to room temperature, approximately 10-15 minutes.
05 - Gently fold whipped topping or whipped cream into the watermelon mixture using a spatula. Use light, sweeping motions to maintain airiness until fully combined and smooth.
06 - Pour filling into the chilled graham cracker crust. Smooth top with spatula. Refrigerate for at least 4 hours or until filling is completely set and firm.
07 - Top pie with additional whipped cream, fresh watermelon balls or slices, and mint leaves if desired. Serve chilled and slice with a sharp knife dipped in warm water for clean cuts.