Make Pudding Base: In a large bowl, whisk sweetened condensed milk and cold water until smooth. Add instant pudding mix and whisk for 2 minutes until thickened. Refrigerate for 5 minutes.
Whip Cream: In a separate bowl, beat heavy cream with a hand mixer on high speed until stiff peaks form, about 3-4 minutes.
Add Marshmallow: Gently fold marshmallow fluff into the whipped cream using a rubber spatula.
Combine Mixtures: Fold the whipped cream mixture into the chilled pudding base until smooth and creamy.
Prep Bananas: Slice bananas into quarter-inch rounds.
Layer Pudding: In a trifle dish or 9×9-inch pan, layer vanilla wafers on the bottom. Add a layer of banana slices, then spread pudding mixture on top. Repeat layers, ending with pudding. Crush a few wafers and sprinkle on top.
Chill: Cover with plastic wrap and refrigerate for 2-4 hours before serving.