Stir the condensed soup and water together in a slow cooker until smooth. Add the shell pasta and andouille sausage. Set the slow cooker to High; cook 2 hours, stirring frequently to prevent sticking.
Once the pasta is tender, stir in the sour cream, Cheddar, and mozzarella until the cheeses melt. Season to taste with salt and pepper. Remove from heat, and allow to rest 15 minutes before serving.
