Meanwhile, combine oil, olives, parsley and garlic in a large bowl and season with sea salt and freshly cracked black pepper. Preheat a barbecue, chargrill pan, or frying pan to medium-high. Lightly brush the cut sides of the potatoes with oil, then cook for 2 minutes each side or until charred/browned. Immediately add the potatoes to the olive mixture and toss well to combine. Stir through watercress. Serve warm.