In a small bowl, beat sweetened condensed milk and water together for about 1 minute. Add pudding mix in and beat well, 2 minutes. Cover and refrigerate 4 hours to overnight. You need this mixture very thick before continuing.
Remove pudding mixture from refrigerator and stir in pumpkin and pumpkin pie spice. Set aside.
Whip cream in large bowl until stiff peaks form. Add pumpkin pudding mix to cream by the spoonful, folding in each time. No streaks of cream or pudding mix should remain.
For assembly, select a large bowl, serving dish or trifle bowl with 4 to 5 qt. capacity. Using a large ice cream scoop, add three scoops of pudding to the bottom and spread. Top with a layer of gingersnap cookies and one sliced banana. Layer with one third of remaining pudding, more cookies and one banana. Continue this layering pattern two more times until you have no more cookies or bananas. Top generously with remaining pudding, being sure all bananas are covered. Cover with plastic wrap and refrigerate at least 8 hours or until cookies are completely soft.
Top with more whipped cream and cookies, if desired.
*Libby's is the thickest pumpkin puree on the market and you want to use a thick one. If you have a wet pumpkin puree, you'll want to line a strainer with cheesecloth and drain it well first before adding it into this recipe.