1. In a large saucepan, combine the water and clam broth with the shrimp shells, smashed garlic, sherry, crushed red pepper, bay leaves and one third of the onion. Bring to a boil and simmer, covered, over low heat for 20 minutes. Strain the shrimp stock into a heatproof bowl and discard the solids.
2. In a soup pot, heat the oil. Add the fennel, celery, bell pepper, minced garlic and the remaining onion. Cover and cook over moderate heat, stirring once or twice, until the vegetables are barely softened, 3 minutes. Uncover and cook until tender, 3 minutes longer. Add the tomatoes with their juices and the shrimp stock; bring to a simmer. Add the potato, season with salt and pepper and simmer until just tender, 15 minutes. Add the shrimp, grouper, oysters and Worcestershire sauce; simmer until cooked through, 3 minutes. Off the heat, stir in the buttermilk and parsley. Serve in deep bowls.