Prepare the Cake: Preheat the oven to 350°F (175°C) and grease and flour two 9-inch round cake pans. Prepare the cake mix according to the box instructions, using eggs, butter, and water. Pour the batter evenly into the pans. Bake for 25-30 minutes or until a toothpick comes out clean. Let the cakes cool completely.
Make the Ambrosia Filling: While the cake is cooling, whip the heavy cream until stiff peaks form (or use store-bought whipped topping). Gently fold in the shredded coconut, mini marshmallows, pineapple, and mandarin oranges until well combined.
Assemble the Cake: Once the cake is cool, slice each cake in half to create four layers. Place the first layer on a platter and spread a layer of ambrosia filling on top. Repeat with the remaining layers and filling.
Finish the Cake: Spread the remaining ambrosia filling on top of the cake. Garnish with shredded coconut or maraschino cherries if desired. Chill the cake in the refrigerator for at least 2 hours.
Serve and Enjoy: After chilling, slice and serve cold. Enjoy!