01 - Combine graham cracker crumbs and sugar in a bowl. Stir in melted butter until fully incorporated. Press crumb mixture firmly into the bottom and up the sides of a 9-inch pie dish. Refrigerate for at least 30 minutes to set.
02 - Puree seedless watermelon in a blender until smooth. Strain through a fine-mesh sieve to remove excess pulp for a smoother texture.
03 - Sprinkle gelatin over cold water in a small saucepan. Let stand for 5 minutes to bloom. 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. Stir in the dissolved gelatin until well combined. Let the mixture cool to room temperature.
05 - Gently fold whipped topping or whipped cream into the watermelon mixture until fully combined and smooth. Be careful not to deflate the mixture.
06 - Pour the filling into the chilled crust. Smooth the top with a spatula. Refrigerate for at least 4 hours, or until completely set.
07 - Before serving, top with additional whipped cream, fresh watermelon balls or slices, and mint leaves if desired. Slice and serve chilled.