01 - Combine graham cracker crumbs and sugar in a mixing bowl. Pour in melted butter and stir until mixture resembles wet sand. Press firmly into bottom and sides of 9-inch pie dish. Refrigerate for 30 minutes to set.
02 - Blend seedless watermelon until completely smooth. Strain through fine mesh sieve to remove excess pulp if desired. Set aside.
03 - Sprinkle gelatin powder over cold water in small saucepan. Let stand 5 minutes to bloom. Heat gently over low heat, stirring constantly until gelatin dissolves completely. Do not boil.
04 - Whisk together watermelon puree, sugar, and lemon juice in large bowl. Stir in dissolved gelatin until fully incorporated. Let mixture cool to room temperature.
05 - Gently fold whipped topping or whipped cream into watermelon mixture until no white streaks remain. Keep folding until smooth and uniform in color.
06 - Pour filling into chilled crust. Smooth top with spatula. Refrigerate at least 4 hours or overnight until completely set.
07 - Top with additional whipped cream if desired. Arrange fresh watermelon balls or slices and mint leaves on top. Slice and serve chilled.