Cut the seedless watermelon into large chunks. Put half of the watermelon chunks into a blender and blend until liquified. Pour through a strainer into a large bowl, using a spatula to push the watermelon against the surface of strainer so that all the juice is removed. Discard the pulp and repeat with the second half of the watermelon. Add the sugar to the juice and stir until dissolved. Refrigerate for roughly 2 hours. Once chilled, pour the juice into a large pitcher. Add the beer and maraschino liqueur and stir to combine. Pour into pint glasses. Makes 4 servings.