Step 1: Prepare the Boil: In a very large stockpot (at least 16-quart capacity), combine water, Old Bay seasoning, lemon juice, lemon halves, garlic, onion, bay leaves, black peppercorns, cayenne pepper, and salt. Bring the mixture to a rolling boil over high heat. Let it boil for 10-15 minutes to allow the flavors to meld.
Step 2: Add Potatoes and Corn: Add the halved red potatoes to the boiling liquid. Reduce heat slightly and cook for 10-12 minutes, or until the potatoes are just tender when pierced with a fork but not mushy. Then, add the corn on the cob pieces and cook for another 5 minutes.
Step 3: Cook Crab and Shrimp: Add the crab legs to the pot. Cook for 5-7 minutes, depending on size, or until heated through and fragrant. Next, add the shrimp and cook for 2-3 minutes, or until they turn pink and opaque. Be careful not to overcook the shrimp, as they can become tough.
Step 4: Add Mussels: Finally, add the scrubbed and debearded mussels. Cover the pot and cook for 3-5 minutes, or until all the mussels have opened. Discard any mussels that do not open.
Step 5: Drain and Serve: Carefully drain the entire contents of the pot into a large colander. You can reserve some of the cooking liquid if desired for dipping, though melted butter is traditional. Transfer the seafood, potatoes, and corn onto a large, newspaper-lined table or serving platter.
Step 6: Garnish and Enjoy: Drizzle with melted butter, sprinkle with fresh chopped parsley, and serve immediately with extra lemon wedges on the side. Provide plenty of napkins and bowls for shells. Enjoy your delicious seafood boil recipe!