Bring a pot of salted water to a boil, add pasta, and cook until al dente, about 8-10 minutes. Drain and rinse under cold water.

If using fresh shrimp, simmer in a separate pot until pink and opaque, about 2-3 minutes. Chop into bite-sized pieces. If pre-cooked, just chop.

Dice celery, red bell pepper, and red onion while pasta cools. Soak onion in cold water if desired.

Whisk together mayonnaise, sour cream, Dijon mustard, lemon juice, and Old Bay seasoning in a medium bowl until smooth. Season with salt and pepper.

In a large bowl, combine cooled pasta, shrimp, imitation crab, and diced vegetables. Pour dressing over and gently toss to coat.

Cover and refrigerate for at least 1 hour before serving to meld flavors.

Serve chilled and garnish with parsley if desired.