Preheat the oven to 350 degrees F. Line a 13-by-9-inch baking pan with foil, leaving a 2-inch overhang on the long sides of the pan. Spray the foil with cooking spray.

Mix the melted butter into the finely ground vanilla wafers and add 2 tablespoons of water. Press the crumbs onto the bottom of the prepared pan, forming a crust. Bake the crust for 20 minutes and allow to cool.

Set aside 2 cups of the watermelon juice in a small bowl. In a large bowl, combine the remaining 3 cups of watermelon juice with the lemon zest, lemon juice, sugar, and heavy cream.

Sprinkle the gelatin over the reserved 2 cups of watermelon juice. Allow to sit for 2 minutes to bloom. Heat in the microwave for 2 minutes on high, or gently in a pot over low heat, until the gelatin has completely melted.

Add the watermelon juice with the melted gelatin to the larger bowl of juice. Stir to combine. Gently pour the mixture over the prepared crust and place in the freezer for 2 hours or until firm. Alternatively, refrigerate for 3 to 4 hours.

Keep in the refrigerator until ready to serve. Cut into squares before serving. If desired, sprinkle with confectioners' sugar or add a dollop of whipped cream.