Beat together the pudding mix, condensed milk, and water in a medium bowl with a handheld electric mixer until combined, about 2 minutes. Cover and refrigerate the pudding mixture for 3-4 hours, or overnight, before continuing. This step is very important to allow the pudding to set properly!
In the bowl of a stand mixer, whip the heavy whipping cream with the whisk attachment until stiff peaks form. Gently fold the pudding mixture into the whipped cream mixture, scraping the bottom of the bowl to ensure it is fully combined and no streaks of pudding remain.
To assemble the pudding: Lightly grease a 4-5 quart dish. I used a rectangular porcelain one. Arrange one third of the wafer cookies onto the bottom of the dish, then one third of the bananas, and then finally one third of the banana pudding. Repeat layers twice, finishing with the banana pudding. Crush any remaining wafers on top of the pudding layer.
Cover very tightly and refrigerate for 4 hours, or up to 8 hours, before serving. This is best served within 2 days.