Place water, salt, and shrimp and crab boil in a 5-gallon stockpot. Bring water to a boil. Add potato halves and onions; return to a boil. Cover stock pot, and cook 20 minutes.
Add garlic, lemons, vinegar, shrimp, crabs, and corn; cook 10 minutes. Remove from heat; cool in pot 10 minutes. Drain carefully.
Transfer seafood and vegetables to a newspaper-covered picnic table. Serve with melted butter and Seafood Cocktail Sauce.