Bring the water to a boil in a very large stockpot. Add the Old Bay, salt, garlic, chile flakes and parsley and boil for 10 minutes to infuse. Add the potatoes and cook another 10 minutes. Add the crabs; cook 10 minutes more, or until the crabs are cooked through. Carefully remove the crabs with tongs; then add the shrimp and cook for 2 to 3 minutes, or until just opaque. Drain immediately. Serve the crab, shrimp and potatoes with melted butter, toasted bread and lemon wedges.