Begin by selecting a perfectly ripe watermelon. Cut it into cubes, ensuring to remove all seeds, and place them aside in a bowl.

In a chilled mixing bowl, pour in the heavy whipping cream, sweetened condensed milk, fresh lime juice, and vanilla extract. Using an electric mixer, whip the mixture on medium speed until soft peaks form, about 3-4 minutes.

Gently fold in the prepared watermelon cubes using a spatula, being careful to maintain the fluffy texture.

Pour the watermelon filling into the pre-made graham cracker crust and smooth the top with a spatula.

Cover the pie with plastic wrap and refrigerate for at least 4 hours, or overnight for best flavor.

Once set, remove the pie from the refrigerator and optionally garnish with fresh mint leaves and mixed berries. Slice and serve.