Place the shells in a small pot with the water. Bring to a boil, reduce the heat to low and simmer, uncovered, for about 15 minutes. Strain the liquid and discard the shells.Meanwhile, in a medium pot over medium heat, melt the butter.
Add the onion, garlic and carrots. Cook until soft, about 2 minutes.
Add the flour and season with salt and pepper. Stir and cook for 2 minutes more. Gradually whisk in the warm milk, reduce the heat to medium-low and continue cooking for about 3 minutes.
Add the mashed potatoes and stir until combined.
Add the shrimp water and cook for about 5 minutes more.
Add the shrimp, cream, wine, salt and pepper. Cook for 4 minutes more and adjust the seasonings to your taste.Ladle the soup into bowls and garnish with cilantro or parsley.