Prepare the Seafood: If using raw shrimp, cook them in boiling water for 2-3 minutes until pink and opaque. Drain, cool, and chop. If using pre-cooked shrimp, simply chop into bite-sized pieces.
Chop the Vegetables: Dice the celery, red bell pepper, red onion, and parsley.
Make the Dressing: In a large bowl, mix mayonnaise, Dijon mustard, lemon juice, Old Bay seasoning, garlic powder, black pepper, salt, paprika, and Worcestershire sauce. Stir until smooth.
Combine Everything: Add the chopped shrimp, crab meat, and vegetables to the dressing. Toss gently to coat.
Chill & Serve: For the best flavor, cover and refrigerate for at least 30 minutes before serving. Serve on a bed of lettuce, in a sandwich, or with crackers.