1 cup drained canned Italian peeled tomatoes, chopped
1/4 cup finely chopped flat-leaf parsley
3/4 pound thick, firm white-fleshed fish fillets, such as snapper, halibut or sea bass, cut into 2-inch pieces
1/2 pound large shrimp, peeled and deveined
Salt and freshly ground pepper
1 cup water
Preheat the oven to 350°. Toast the bread directly on the oven rack, turning once, until golden and crisp, about 8 minutes. Rub the toast with the peeled garlic clove and drizzle with 2 tablespoons of the olive oil. Turn off the heat and keep the toast warm in the oven with the door ajar.
In a large saucepan, heat the remaining 1/4 cup of olive oil until shimmering. Add the onion and the minced garlic and cook over moderate heat, stirring, until golden, about 4 minutes. Add the mussels, cockles and wine and cook over high heat, stirring frequently, until most of the shells have opened and the liquid has reduced by half, 4 minutes. Add the chopped tomatoes and 3 tablespoons of the parsley, cover and bring to a boil.
Add the fish and shrimp and season with salt and pepper; bring to a boil. Add the water to slow the cooking. Simmer over moderately low heat until cooked through, 7 minutes. With a slotted spoon, transfer the seafood to a large, deep platter, cover and keep warm.
Boil the broth until reduced by one-fourth, 5 minutes, then pour it over the fish stew. Season the stew with salt and pepper, spoon into bowls and sprinkle with the remaining 1 tablespoon of parsley. Serve hot, with the garlicky toast.