Combine the shrimp boil seasoning, beer, onion, and halved lemon with 5 quarts of water in a large stockpot. Bring to a boil and boil for 15 minutes to develop the flavors.
Meanwhile, cut the sausage into 3-inch lengths and the corn into thirds.
Add the sausage and potatoes to the boiling water. Cover and cook 10 minutes. Uncover and add the corn. Cover and cook 5 minutes.
Uncover and add the shrimp. Cover and cook just until the shrimp turn pink, about 2 minutes.
Remove from the heat and let stand for 5 minutes. Drain the seafood boil and serve with butter, cocktail sauce, tartar sauce, and lemon wedges.