Melt the butter in a saucepan over low heat. Sauté the onion and celery in it until they turn tender. Then, add the flour, marjoram, thyme, and cayenne pepper to the pan, and stir until the mixture turns smooth. Next, add the potato soup and milk to the pan, and let the mixture simmer. In the meantime, clean and cut the seafood. Add it to the pan and let it cook over medium heat. Ladle the chowder into bowls, garnish it with fresh parsley, and serve it with a salad and bread on the side.