In Dutch oven, combine 6 cups water, salt and bag of crab boil mixture; cover tightly. Bring to a boil. Add potatoes and corn; return to a boil. Reduce heat to medium; cover and cook 10 to 15 minutes or just until potatoes are tender and corn is bright yellow.
Remove Dutch oven from heat. Add shrimp; let stand 3 to 5 minutes or until shrimp turn pink.
Drain potatoes, corn and shrimp; discard bag of seasoning. Place corn and potatoes on serving platter; drizzle with melted margarine. Arrange shrimp on platter; squeeze lemon wedges over shrimp. If desired, sprinkle with freshly ground black pepper and chopped fresh parsley; serve with cocktail sauce.