Place potatoes in a large pot; cover with cold salted water. Cover and bring to a boil, reduce heat to low, and simmer until tender, 15 to 18 minutes. Cut into halves or quarters, if large. Meanwhile, whisk ­together oil, vinegar, Dijon, and pickle brine in a bowl. Season with salt and pepper. Add potatoes, pickles, and mustard seeds, and toss to coat. Fold in watercress, chives, and parsley.