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 - Puree seedless watermelon in blender until smooth. Strain through fine mesh sieve to remove excess pulp if desired.
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.
04 - In large bowl, mix watermelon puree, sugar, and lemon juice. Stir in dissolved gelatin until well combined. Let mixture cool to room temperature.
05 - Fold whipped topping or whipped cream into watermelon mixture until fully combined and smooth, taking care not to deflate the mixture.
06 - Pour filling into chilled crust. Smooth top with spatula. Refrigerate for at least 4 hours or until completely set.
07 - Top with additional whipped cream, fresh watermelon balls or slices, and mint leaves if desired. Serve cold.