1. Heat a large cast-iron skillet over high heat. Add scallops to pan; cook 1 minute or until browned. Turn over and cook 30 seconds. Place scallops in a bowl.
2. Combine vinegar and next 4 ingredients (through pepper), stirring with a whisk. Pour vinegar mixture over scallops. Cover and refrigerate 30 minutes, stirring occasionally. Arrange 1/2 cup watercress on each of 6 salad plates. Remove scallops from marinade with a slotted spoon, reserving marinade. Divide scallops evenly among plates. Drizzle each serving with about 1 tablespoon marinade.