• Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.
• Step 2: In a large mixing bowl, combine the white cake mix, pistachio pudding mix, vegetable oil, eggs, and water. Beat on medium speed for about 2 minutes, scraping down the sides as needed, until the batter is smooth and well-combined.
• Step 3: Fold in the chopped walnuts, if using, until evenly distributed throughout the batter.
• Step 4: Pour the batter into the prepared baking pan and spread it evenly.
• Step 5: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan on a wire rack.
• Step 6: Once the cake has cooled, spread the whipped topping evenly over the top. Sprinkle the drained crushed pineapple over the whipped topping, covering the entire surface.
• Step 7: Refrigerate the Watergate Cake for at least 2 hours before serving to allow the flavors to meld and the topping to set.