Bring a large pot of salted water to a full boil. Add rotini and cook 1 minute less than the package al dente time (about 7-8 minutes). Set a timer.
Drain immediately. Rinse under cold running water for 90 seconds until completely cool. Shake off excess water and transfer to a large mixing bowl.
Whisk mayo, sour cream, Old Bay, lemon juice, Dijon, celery salt, and pepper until smooth. Taste — it should be bold.
Shred imitation crab into bite-sized pieces. If using shrimp, thaw fully and pat dry. Both must be cold before mixing.
Finely dice celery and red onion. Thaw peas under cold water. Chop fresh dill.
Add celery, onion, and 2/3 of the peas to the pasta. Pour dressing over and toss to coat. Fold in seafood gently.
Set aside remaining peas and half the dill for serving.
Cover and refrigerate at least 30 minutes. Before serving, taste and adjust seasoning. Add reserved peas and dill. Serve cold.
