Thaw the shrimp if using frozen; hold under cold water for 5 minutes. Pat dry and devein.

Slice the cucumbers thinly and chop the red onion finely. Soak the onion in ice water for 10 minutes, then drain.

In a bowl, whisk together mayonnaise, sour cream, lemon juice, dill, salt, and pepper.

In a large bowl, combine the cucumbers, soaked onions, and shrimp. Pour the dressing over, then gently toss.

Cover and refrigerate for at least 30 minutes before serving.