Tapioca thickens the filling in this vibrant and festive watermelon pie. A sweet watermelon flavor shines in this cooling pie with whipped cream and orange zest that elevates the fruitiness. This pie is perfect for a hot summer day.
Lightly coat a 9-inch pie pan with cooking spray. Place graham cracker crumbs and 2 tablespoons granulated sugar in a food processor; process until well combined, about 5 seconds. Add butter; process until evenly combined, 5 to 10 seconds. Press the mixture into the bottom and up the sides of the prepared pan. Freeze, uncovered, until hardened, at least 30 minutes or up to 12 hours.
Preheat oven to 325°F. Bake the frozen pie crust until lightly browned, 8 to 12 minutes. Let cool completely on a wire rack, about 30 minutes.
Cut watermelon in half; remove and discard rind. Refrigerate 1 watermelon half until ready to use. Chop the remaining watermelon half into cubes (about 3/4-inch) to equal 3 cups; (reserve any remaining chopped watermelon for another use). Place the watermelon cubes in a blender; process until completely smooth, about 30 seconds. Pour through a fine-mesh strainer into a medium bowl, pressing on the pulp in the strainer to squeeze out juices; discard solids. Measure watermelon juice to equal 1 1/3 cups and set aside (reserve any remaining juice for another use).
Process tapioca in a spice grinder until finely ground, about 30 seconds (alternatively, grind tapioca using a mortar and pestle). Transfer the ground tapioca to a small saucepan; whisk in the watermelon juice, salt and the remaining 1/3 cup granulated sugar. Let stand for 5 minutes. Bring to a boil over medium-high heat, whisking constantly. Boil, whisking constantly, until the mixture is thickened and the tapioca is mostly translucent, about 1 minute. Remove from heat and let cool, stirring occasionally, for 15 minutes. Stir in orange juice; set aside.
Beat whipping cream with an electric mixer fitted with a whisk attachment on medium-high speed until soft peaks form, about 1 minute. Add vanilla and confectioners' sugar, 1 tablespoon at a time, beating until stiff peaks form, about 30 seconds. Transfer 1/2 cup of the whipped cream to a small bowl; cover and refrigerate until ready to use. Gently fold the cooled watermelon juice mixture into the remaining whipped cream, about 1/4 cup at a time, until smooth and combined. Spoon the mixture into the cooled pie crust. Refrigerate, uncovered, until firm, at least 8 hours or up to 12 hours.
Using a melon baller, scoop out flesh from the reserved watermelon half to make 18 (about 3/4-inch) balls. Spread the reserved whipped cream over the center of the pie, leaving a 1 1/2-inch border; arrange the watermelon balls around the outer edges. Sprinkle orange zest over the whipped cream. Serve immediately.
Freeze crust (Step 1) for up to 12 hours. Prepare the pie through Step 5 and refrigerate for up to 12 hours. Continue with Step 6 just before serving.