Quarter potatoes and place in a medium saucepan, cover with water, and place over high heat to bring water to a boil. Boil potatoes until just tender, 10 minutes. Drain and return to warm sauce pan. Drizzle potatoes with a touch of oil and let stand 5 minutes in warm pot to dry them out.
Transfer cooked potatoes to a bowl. Combine yogurt, oil, and horseradish and pour over potatoes. Coarsely chop the leafy tops of watercress and add to the mixing bowl with potatoes. Sprinkle a little coarse salt over the bowl and toss salad to coat evenly. Adjust salt to your taste and serve at room temperature.