In a large bowl, stir together 20 ounces crushed pineapple and 6.8 ounces instant pistachio pudding mix until combined.
Fold in 2 cups miniature marshmallows and 3/4 cup pistachios. Set aside.
Beat the cold 1 1/2 cups heavy whipping cream with the 1/4 cup powdered sugar using an electric hand mixer or a stand mixer with the whisk attachment. Beat on high speed until medium peaks form and it holds its shape. Use a rubber spatula to transfer the mixture to a separate bowl.
Add the softened 4 ounces cream cheese to the mixing bowl and beat until soft and fluffy.
Use the spatula to scrape it away from the sides of the bowl. Add about 2/3 of the sweetened whipped cream to the cream cheese (the remaining whipped cream will be reserved as topping when serving). Whip the mixture together until light and fluffy.
Fold the cream mixture into the pistachio mixture. Cover and refrigerate for at least 3 hours.
Serve individual portions topped with the reserved whipped cream and chopped pistachios.
Notes
Watergate Salad can be made in advance and stored in an airtight container for 2-3 days. Just remember to cover it with plastic wrap and stir well before serving.