Dice the celery and onion finely, chop the shrimp if the pieces are large, and gently chunk the crab so it keeps some texture.
Pat the seafood dry well with paper towels before mixing so the dressing does not turn watery.
In a medium bowl, whisk together the mayonnaise, sour cream, lemon juice, Dijon mustard, Old Bay, and black pepper until smooth.
Taste the dressing and adjust with a little more lemon juice or seasoning only if needed.
Add the shrimp, crab, celery, and onion to the bowl and fold gently until evenly coated.
If the salad feels too thick, loosen it with another spoonful of sour cream or a small amount of lemon juice rather than water.
Cover and chill for at least 30 to 60 minutes so the flavor comes together and the texture firms up.
Taste again before serving, then adjust salt and lemon as needed.
Serve on crackers, in lettuce cups, over greens, or in a toasted roll.
