In a 12- to 16-quart pot bring 2 gallons water to boiling. Add shrimp and crab boil and seafood seasoning; reduce heat. Simmer, uncovered, 10 minutes.
Add potatoes, a few at a time, allowing water to continue simmering. Simmer 7 to 10 minutes, until nearly tender.
Add corn; simmer, uncovered, 5 minutes.
Add sausage and shrimp. Simmer 3 to 5 minutes, until shrimp are opaque (do not overcook shrimp) and sausage is heated through; drain.
Transfer to large platter. Add lemon halves and sprinkle parsley over top. Serve hot or within 1 hour of cooking. Pass Garlic Mayonnaise and Cocktail Sauce. Makes 6 to 8 servings.