How to Make It
In a large skillet, heat 1 tablespoon of the olive oil until shimmering. Add the shrimp, season with salt and pepper and cook over high heat, stirring occasionally, for 1 minute. Add 1/2 teaspoon of the garlic and cook, stirring, until the shrimp are opaque, about 1 minute. Transfer the shrimp to a platter. Add the water to the skillet and cook over high heat, scraping up any browned bits stuck to the bottom of the pan. Pour the pan juices into a bowl and wipe out the skillet.
Heat 1 tablespoon of olive oil in the skillet. Add the squid and scallops. Season with salt and pepper and cook, stirring, for 1 minute. Add 1/2 teaspoon of the garlic and cook just until fragrant, about 1 minute longer. Transfer to the platter.
Pour the reserved pan juices from the bowl into the skillet and cook over high heat, scraping up any browned bits stuck to the bottom of the pan. Add the mussels, cockles and the remaining 1/2 teaspoon of garlic and bring to a boil. Cover and cook over high heat until the shells open, about 5 minutes. Using a slotted spoon, transfer the mussels and cockles to a large bowl; discard any that do not open. When the shells are cool enough to handle, remove the meat and add it to the platter.
Pour any accumulated shellfish juices into the skillet and bring the liquid to a boil. Simmer over moderate heat until reduced to 1/4 cup. Pour the liquid into a large bowl. Add the lemon juice and the remaining olive oil and let cool. Stir in all of the seafood and let marinate in the refrigerator for 30 minutes.
Meanwhile, in a medium saucepan of boiling salted water, cook the beans until crisp-tender, about 5 minutes. Drain and cool under running water; pat dry. Add the beans, basil and parsley to the seafood salad. Season with salt and cayenne and serve with lemon wedges.