In a large stockpot, bring water to a boil. Add Cajun seasoning, paprika, cayenne pepper, lemon halves, garlic, and onion. Stir well.
Reduce heat to a simmer and let the broth cook for 10 minutes to allow the flavors to develop.
Add potatoes to the pot and cook for 10 minutes.
Add corn and sausage slices, cooking for an additional 5 minutes.
Gently stir in shrimp and crab (if using), cooking until shrimp are pink and opaque, about 3–5 minutes.
Drain the pot, reserving some broth for dipping if desired. Spread the seafood boil onto a large serving tray or newspaper-lined table for communal dining.
Drizzle with melted butter, sprinkle extra Cajun seasoning, and garnish with fresh parsley. Serve immediately with dipping sauces like garlic butter or Cajun seafood sauce.