Prepare the Boiling Liquid:
In a large stockpot, combine water, beer (if using), Old Bay seasoning, lemon halves, salt, sugar, black peppercorns, and bay leaves. Bring to a boil over high heat.
Cook the Potatoes and Corn:
Add the baby red potatoes to the boiling liquid and cook for 10-12 minutes, or until just tender.
Add the corn and sausage pieces to the pot and cook for another 5 minutes.
Add the Seafood:
Add the shrimp, crab legs, clams, mussels, and minced garlic to the pot. Cook for 5-7 minutes, or until the shrimp are pink and the clams and mussels have opened. Discard any unopened clams or mussels.
Serve:
Using a strainer or slotted spoon, carefully transfer the seafood, potatoes, corn, and sausage to a large serving platter or spread out on newspapers for a rustic presentation.
Drizzle the melted butter over the seafood boil and garnish with chopped fresh parsley.
Serve with lemon wedges on the side.