In a medium bowl, combine graham cracker crumbs, 2 tablespoons granulated sugar, and melted butter. Mix until the crumbs are evenly moistened. Press the mixture firmly and evenly into the bottom and up the sides of a 9-inch pie plate. Chill the crust in the refrigerator for at least 15 minutes while you prepare the filling.

Place the cubed watermelon into a blender or food processor and blend until completely smooth. Pour the watermelon puree through a fine-mesh sieve set over a large bowl, pressing lightly on the solids to extract as much juice as possible. You should have about 3 cups of strained watermelon juice. Discard the pulp.

In a small microwave-safe bowl, sprinkle the gelatin packets over the cold water. Let it sit for 5 minutes to bloom (it will thicken and get spongy). Microwave the bloomed gelatin for 15-20 seconds, or until completely dissolved and clear, stirring once. Do not boil.

Add the 0.5 cup granulated sugar and lime juice to the strained watermelon juice. Stir until the sugar is dissolved. Pour the dissolved gelatin mixture into the watermelon juice, stirring constantly for about 1 minute to ensure it's fully incorporated. Place this mixture in the refrigerator for 20-30 minutes, stirring occasionally, until it starts to thicken slightly but is not yet set.

In a separate large, very cold bowl (chilling the bowl and beaters helps!), beat the heavy cream with the powdered sugar and vanilla extract using an electric mixer on medium-high speed until stiff peaks form. Be careful not to overbeat.

Once the watermelon mixture has slightly thickened, gently fold about one-third of the whipped cream into it to lighten it, then fold in the remaining whipped cream until just combined, being careful not to deflate the mixture. Pour the watermelon filling into the chilled graham cracker crust, spreading it evenly.

Cover the pie loosely with plastic wrap and refrigerate for at least 4-6 hours, or preferably overnight, until the pie is completely set. Before serving, garnish with fresh mint leaves if desired. Slice with a sharp knife, wiping the knife clean between slices for neat cuts.