In a large skillet, melt the butter over moderate heat. Add the onion, carrot, celery and garlic and cook until just softened, about 5 minutes. Add the corn, bay leaf and saffron and cook for 5 minutes, stirring occasionally. Add the water and bring to a boil. Reduce the heat to moderately low and simmer for 15 minutes.
Discard the bay leaf. Transfer the soup to a blender and puree. Strain the soup through a fine sieve into a large bowl. Season with salt and pepper. Ladle the soup into bowls, top with the crabmeat and basil and serve.