In a large bowl, whisk together sweetened condensed milk and ice-cold water until smooth.
In a separate bowl, add instant vanilla pudding mix and slowly whisk in the milk mixture. Whisk until no lumps remain and mixture is smooth, about 1 minute. Cover and refrigerate until firm, at least 1 hour or overnight.
In a stand mixer with whisk attachment, whip cold heavy cream on medium speed for 1 minute until it begins to thicken. Increase to medium-high and whip until stiff peaks form. Do not over whip.
Gently fold the chilled pudding into the whipped cream on low speed until fully blended with no streaks remaining.
To assemble, spread one-quarter of the pudding in the bottom of a trifle bowl or 4-5 quart glass bowl. Layer with one-third of the Nilla Wafers and one-third of the sliced bananas.
Repeat layering twice more, ending with a final layer of pudding on top. Garnish with additional wafers if desired.
Cover tightly with plastic wrap and refrigerate for 4 to 6 hours before serving. Wafers should be tender when poked with a knife. Best served within 12 hours of assembling.
