In a medium-sized bowl, combine the sweetened condensed milk, ice-cold water, and instant vanilla pudding mix. Whisk or use a hand mixer until well combined, about 2 minutes. Cover the bowl and refrigerate for 3-4 hours, or preferably overnight, until the pudding is set.

In a separate large bowl, whip the heavy cream until stiff peaks form. You can use a hand mixer or a stand mixer for this step. Gently fold the whipped cream into the set pudding mixture until just combined.

To assemble, spread one-third of the Nilla Wafers in the bottom of a large bowl. Top with one-third of the sliced bananas, followed by one-third of the pudding mixture. Repeat these layers two more times. You can garnish the top with extra wafer crumbs if desired.

Cover the bowl tightly with plastic wrap and chill for at least 4 hours before serving. It's best not to chill for longer than 8 hours.