01 - Combine graham cracker crumbs and sugar in a bowl. Pour in melted butter and stir until fully incorporated. Press crumb mixture firmly into the bottom and up sides of a 9-inch pie dish. Refrigerate for 30 minutes to set.
02 - Blend seedless watermelon in a blender until completely smooth. Strain through a fine-mesh sieve to remove excess pulp if a smoother texture is desired.
03 - Sprinkle gelatin powder 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 - Whisk together watermelon puree, sugar, and lemon juice in a large bowl. Stir in dissolved gelatin until fully incorporated. Let mixture cool to room temperature, approximately 10-15 minutes.
05 - Gently fold whipped topping or whipped cream into the watermelon mixture using a spatula. Continue folding until fully combined and smooth, being careful not to deflate the mixture.
06 - Pour filling into chilled pie crust. Smooth top with spatula. Refrigerate for at least 4 hours or until completely set and firm to the touch.
07 - Top with additional whipped cream, fresh watermelon balls or slices, and mint leaves just before serving. Slice and serve chilled.