Roughly chop the imitation crab into pieces that are bite-size. Do the same thing if you have larger cooked shrimp. Set aside.
In a large mixing bowl, mix together the mayonnaise, lemon juice, Old Bay, salt, and black pepper until smooth.
Add the finely diced celery, red onion, and chopped fresh dill to the dressing. Mix together well.
Add the chopped crab meat and cooked shrimp. Gently stir the ingredients with a spatula until the seafood is fully coated in the dressing.
Cover the bowl with plastic wrap and place in the fridge. Let the salad cool for at least 1 hour before serving.
Serve chilled with croissants, crackers, or on a bed of fresh greens.