1. Heat water, lemon, and peppercorns in large saucepan to boiling. Add scallops and cook over medium heat 2 minutes. With slotted spoon, transfer scallops to large bowl. Return liquid to boiling; add shrimp and cook covered 4 minutes over medium heat. With slotted spoon, transfer shrimp to colander. Cool.
2. Meanwhile, reheat liquid to boiling; add squid and cook until squid is just opaque, 1 minute. Add squid to scallops. Reheat liquid to boiling; add mussels and cook covered over high heat 2 minutes. Transfer opened mussels to plate. Cover and cook any remaining mussels 1 minute more, transferring opened mussels to plate. Discard any unopened mussels.
3. Peel and devein shrimp; add to bowl with scallops. Remove mussels from shells; add to bowl with scallops.Make Dressing::
4. Stir all dressing ingredients together in small bowl.
5. Add Dressing and olives to seafood mixture and toss to coat. Cover and refrigerate at least 1 hour.
6. At serving time, stir in parsley. Transfer to serving platter and garnish with lemon wedges. Makes 12 servings.