01 - Combine graham cracker crumbs and sugar in a mixing bowl. Pour in melted butter and stir until fully incorporated and mixture resembles wet sand. Press firmly into bottom and up sides of a 9-inch pie dish. Refrigerate for 30 minutes to set.
02 - Blend seedless watermelon in blender until completely smooth. Strain through fine-mesh sieve to remove excess pulp if smoother texture is desired. Set aside.
03 - Sprinkle gelatin powder over cold water in small saucepan. Let stand for 5 minutes to bloom. Place over low heat, stirring constantly until gelatin completely dissolves. Remove from heat.
04 - Whisk together watermelon puree, sugar, and lemon juice in large bowl. Gradually stir in dissolved gelatin mixture. Let cool to room temperature, approximately 10-15 minutes.
05 - Gently fold whipped topping or whipped cream into watermelon mixture using spatula until fully combined and smooth. Be careful not to overmix and deflate mixture.
06 - Pour filling into chilled crust. Smooth top with spatula. Refrigerate for minimum 4 hours or overnight until completely set and firm.
07 - Top with additional whipped cream if desired. Arrange fresh watermelon balls or slices and mint leaves on top. Serve cold.