Place the potatoes and 1 tablespoon of the salt in a medium saucepan and cover with cold water. Bring to a boil, reduce heat to a simmer, and cook for 20 minutes or until the potatoes are tender. Drain and set aside. Place the chicken in a large bowl and set aside. Using a whisk, combine the mustard, vinegar, garlic, and honey in a medium bowl. Add the remaining salt and the pepper. While whisking, gradually add the oil in a thin, steady stream until a medium-thick vinaigrette is formed. Halve the potatoes and combine with the chicken. Add the capers and vinaigrette, gently tossing with the chicken mixture. Fold in the watercress just before serving.