In a large glass bowl, mix the sweetened condensed milk with ice-cold water using a hand mixer for about a minute until fully combined. Add the instant vanilla pudding mix and beat for approximately 2 minutes until the mixture is smooth. Cover the bowl and refrigerate for 3 to 4 hours or overnight, allowing the pudding mixture to set completely. Proper chilling time is crucial to achieve the right consistency; if not chilled long enough, the mixture will be too thin.

In a separate large bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the chilled pudding mixture until fully incorporated and no streaks remain.

For assembly, choose between individual servings or use a large 4-5 quart glass bowl or a 9x13-inch baking dish. Start by arranging 1/3 of the Nilla wafers on the bottom, overlapping them if necessary. Follow with 1/3 of the banana slices and then 1/3 of the pudding mixture. Repeat the layers two more times, finishing with a layer of wafers or wafer crumbs on top.

Cover the assembled dessert tightly with plastic wrap and refrigerate for at least 4 hours or up to 8 hours. Do not chill for longer than 8 hours, as the bananas may begin to brown.