To prepare crust, combine crumbs, margarine, and sugar in a mixing bowl. Press mixture firmly in a 9" pie pan. Chill one hour, or until firm. Or, bake in a 375 oven for ten minutes or until edges are brown. Cool on rack before filling. To prepare filling, combine whipped topping, watermelon gelatin powder, and water. Fold in watermelon balls. Spoon into prebaked crust. Chill for two hours.