01 - Combine graham cracker crumbs and sugar in a bowl. Pour in melted butter and stir 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 completely smooth. Strain through fine mesh sieve to remove excess pulp if desired for a silkier texture.
03 - Sprinkle gelatin evenly over cold water in 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 large bowl. Gradually stir in dissolved gelatin until well incorporated. Allow mixture to cool to room temperature.
05 - Gently fold whipped topping or whipped cream into watermelon mixture 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 until completely set and firm.
07 - Top with additional whipped cream, fresh watermelon balls or slices, and mint leaves if desired. Serve chilled.