1. In a large pot, melt the butter over moderately low heat. Add the onion, celery, garlic, and 3/4 teaspoon of the salt. Cook, stirring occasionally, until the vegetables start to soften, about 10 minutes. Add the wine, tarragon, clam juice, tomatoes, and water. Bring to a boil. Reduce the heat and simmer for 10 minutes.
2. Meanwhile, put a medium frying pan over moderately low heat. Toss the bread cubes with the oil and the remaining 1/4 teaspoon salt; add them to the pan. Cook, stirring frequently, until crisp and golden brown, about 5 minutes. Remove from the heat and toss the croutons with 1/8 teaspoon pepper and 2 tablespoons of the parsley.
3. Stir the cod and shrimp into the soup. Cook, stirring occasionally, until the seafood is just done, 3 to 5 minutes. Stir in 1/4 teaspoon pepper and the remaining parsley and serve the soup topped with the croutons.
Wine Recommendation: Fish soup with an herb-infused broth makes us think of sunlit patios overlooking a shimmering Mediterranean. If you were in Nice, you'd be drinking a Provençal rosé. Why not here?