Arrange banana slices in bottom of piecrust, and spread crushed pineapple over banana. Combine water and pudding mix in a medium bowl; stir with a whisk until mixture is well blended. Fold in 1/2 cup whipped topping.
Spread pudding mixture evenly over fruit; cover and chill at least 2 hours or freeze 45 minutes. Spread remaining whipped topping over pudding mixture. Garnish pie with cherries, if desired.