Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Prepare the cake batter according to the package instructions, mixing together the cake mix, eggs, oil, and water. Pour the batter evenly into the two prepared pans.
Bake for 25-30 minutes, or until a toothpick inserted in the center of the cakes comes out clean. Let the cakes cool in the pans for 10 minutes, then remove from the pans and transfer to a wire rack to cool completely.
While the cake is cooling, prepare the ambrosia filling. In a large bowl, gently fold together the whipped cream, shredded coconut, marshmallows, pineapple, oranges, and nuts (if using). Set aside.
For the frosting, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
Once the cakes are completely cool, spread a layer of ambrosia filling on top of one of the cakes. Place the second cake on top and frost the entire cake with the whipped cream frosting.
Decorate with additional coconut, marshmallows, or nuts as desired.
Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld.