01 - Combine graham cracker crumbs and sugar in a bowl. Stir in melted butter until fully incorporated. Press crumb mixture firmly into bottom and up sides of a 9-inch pie dish. Refrigerate for at least 30 minutes to set.
02 - Blend seedless watermelon in blender until smooth. Strain through fine mesh sieve to remove excess pulp if desired, ensuring smooth consistency.
03 - Sprinkle gelatin over cold water in small saucepan. Let stand for 5 minutes to bloom. Gently heat over low heat, stirring constantly until gelatin dissolves completely. Do not boil.
04 - Whisk together watermelon puree, sugar, and lemon juice in large bowl until sugar dissolves. Stir in dissolved gelatin mixture until well incorporated. Let mixture cool to room temperature.
05 - Gently fold whipped topping or whipped cream into watermelon mixture using spatula until fully combined and smooth, taking care not to deflate mixture.
06 - Pour filling into chilled crust. Smooth top with spatula. Refrigerate for at least 4 hours or overnight until completely set and firm.
07 - Top pie with additional whipped cream, fresh watermelon balls or slices, and mint leaves just before serving. Slice and serve chilled.