In a large bowl, whisk together water, pudding mix, and sweetened condensed milk until smooth. Refrigerate for at least 4 hours, until it sets up completely.

Whip heavy cream until soft peaks form. Working in thirds, fold the whipped cream into the pudding mixture until well incorporated.

In a 3-quart (3-liter) trifle bowl or in individual trifle dishes (12 dishes, 1 cup/240ml capacity), layer wafers, sliced bananas, and pudding mixture, then repeat until you've used up all the pudding. Refrigerate for at least 30 minutes before serving, but no later than 24 hours to prevent sogginess. Leftovers will keep for another 1-2 days, but the cookies will get soggier over time.