01 - Combine graham cracker crumbs and sugar in a bowl. Pour in melted butter, stirring until fully incorporated and mixture resembles wet sand. Press 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 - 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 evenly over cold water in a small saucepan. Let stand for 5 minutes to allow gelatin to absorb the water and bloom. Place over low heat, stirring constantly until gelatin dissolves completely and mixture is clear. Remove from heat.
04 - In a large mixing bowl, whisk together watermelon puree, sugar, and lemon juice. Gradually stir in the dissolved gelatin mixture until fully incorporated. Let cool to room temperature, about 10-15 minutes.
05 - Gently fold whipped topping or whipped cream into the watermelon mixture using a spatula. Fold until no white streaks remain and mixture is smooth and uniform. Do not overmix.
06 - Pour the watermelon filling into the chilled crust. Smooth the top with an offset spatula. Refrigerate for at least 4 hours or overnight until completely set and firm.
07 - Top with additional whipped cream if desired. Decorate with fresh watermelon balls or slices and mint leaves. Slice into wedges and serve chilled.