In a medium bowl, stir together pretzels, brown sugar, flour, and butter until well combined. Firmly press mixture into bottom and up sides of a 9-inch deep-dish pie dish. Bake until set, 15 to 20 minutes. Let cool completely on a wire rack.
In a 1-quart saucepan, stir together 1 cup watermelon juice and gelatin. Let stand for about 2 to 3 minutes. Add granulated sugar; cook over medium heat, stirring constantly, just until sugar and gelatin dissolve. Remove from heat.
In a large bowl, add gelatin mixture, remaining 3 cups watermelon juice, cream, lemon zest and juice. Pour into prepared piecrust and refrigerate 8 hours or overnight. Serve chilled and topped with whipped topping. Garnish with watermelon slices, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/watermelon-pie-recipe/