Preheat grill to medium-high (about 450°F). Soak sliced onion in ice water to cover for 10 minutes; drain well.
Place scallops, olive oil, salt, and pepper in a large bowl, and toss until scallops are evenly coated.
Thread scallops onto the skewers (5 or 6 scallops per skewer). Grill the scallops until grill marks appear, 1 1/2 to 2 minutes on each side. (Overcooking the scallops will make them tough and rubbery.) Transfer skewers to a platter, and drizzle with 1/4 cup of the Citrus Vinaigrette.
Toss together the lettuce, radishes, red onion, and remaining 1/4 cup Citrus Vinaigrette in a large bowl. Divide the salad among four plates, and top each salad with two skewers of the grilled scallops. Sprinkle with sea salt flakes, and serve with orange segments.