In a mixing bowl, combine graham cracker crumbs, melted butter, and sugar. Press the mixture into the bottom of a tart pan.

In a blender, puree the watermelon until smooth. Add lemon juice and blend until combined.

In a separate bowl, fold the whipped cream and powdered sugar into the watermelon mixture until well combined.

Pour the filling into the prepared crust and smooth the top. Freeze for at least 1 hour or until firm.