Mix cucumbers, sweet onion, and sea salt together in a bowl. Cover bowl with plastic wrap and let sit for 30 minutes.
Turn cucumber mixture into a colander set over a bowl or in a sink; let drain, stirring occasionally, until most of the liquid has drained, about 30 minutes more. Transfer drained cucumber mixture to a large bowl.
Stir mayonnaise, vinegar, sugar, dill, garlic powder, and black pepper together in a bowl with a whisk until smooth; pour over the cucumber mixture and stir to coat vegetables with the dressing.
Cover bowl with plastic wrap and refrigerate at least 2 hours.