Whisk the Base: In a large bowl, beat together the sweetened condensed milk and ice-cold water. Add the instant vanilla pudding mix and whisk vigorously for 2 minutes until completely smooth.
First Chill: Cover the bowl with plastic wrap and refrigerate for at least 4 hours (or overnight) until fully set and firm.
Whip the Cream: In a chilled mixing bowl, whip the heavy cream until it forms stiff peaks.
Fold Together: Gently fold the chilled pudding mixture into the whipped cream fraction by fraction using a rubber spatula until the color is uniform and no white streaks remain.
Layer the Dessert: In a large glass trifle bowl or a 9x13-inch baking dish, arrange a single layer of Nilla wafers across the bottom, followed by a layer of sliced bananas, and then one-third of the pudding mixture.
Repeat Layers: Repeat the sequence two more times, ending with a smooth layer of pudding on top. (Optional: Top with crushed wafers for decoration).
Final Set: Cover tightly and refrigerate for a minimum of 4 to 6 hours before serving so the wafers can soften into a cake-like texture. Serve ice-cold!