Pat the shrimp thoroughly dry using paper towels, then chop into bite-sized pieces. Drain crab meat well and inspect for shell fragments if necessary.
Finely dice the celery, cucumber, onion, herbs, capers, and pickles. Grate the additional celery using a fine grater.
In a medium bowl, whisk together the mayonnaise, Dijon mustard, lemon juice, lemon zest, Worcestershire sauce, and hot sauce until smooth and creamy.
In a large bowl, combine the shrimp, crab, celery, grated celery, cucumber, onion, dill, parsley, celery leaves, capers, pickles, Old Bay seasoning, celery seed, garlic powder, black pepper, and cayenne if using.
Add the dressing and gently fold until evenly coated. Avoid overmixing so the seafood stays chunky and tender.
Cover and refrigerate for at least 45 minutes before serving so the flavors fully blend.
Serve chilled with croissants, crackers, sandwich rolls, lettuce wraps, or cucumber slices.