1. For dressing, combine the lemon juice, garlic, salt, and pepper in a medium bowl. Slowly whisk in the oil until blended and set aside.
2. For salad, combine the water, carrot, celery, garlic, peppercorns, and lemon in a medium saucepan. Bring to a boil over high heat. Reduce heat to medium, then add the shrimp, cover and cook 2 minutes. With a slotted spoon, transfer shrimp to a large bowl. Return liquid to a boil, then add the scallops and cook for 2 minutes. Transfer to the bowl with the shrimp. Repeat with the squid, cooking 30 seconds. Return liquid to a boil, then add the mussels and cook, covered, over high heat for 5 minutes. Transfer opened mussels to a plate. Cover and cook remaining mussels 1 minute more. Discard any unopened mussels.
3. Remove mussels from shells and add to bowl with the other seafood. Add the dressing, tossing to coat. Cover and refrigerate at least 1 hour.
4. To serve, stir in the parsley. Garnish with lemon, if desired. Makes 6 servings.