Slice cucumbers into 1/4-inch rounds. Place in a colander, sprinkle with 1/2 teaspoon salt, and let drain for 10 minutes.
Thinly slice red onion into half-rings. Soak in cold water for 5 minutes, then drain well.
In a large bowl, whisk sour cream, mayonnaise, vinegar, sugar, pepper, and garlic powder until smooth.
Pat cucumber slices dry with a clean towel. Add cucumbers, onions, and dill to the dressing. Toss gently to coat.
Taste and adjust seasoning. Serve immediately or refrigerate for 30 minutes before serving.