Place the potatoes and 1 tablespoon of the salt in a medium saucepan. Cover with cold water and 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.
Whisk together the mustard, vinegar, garlic, and honey in a medium bowl. Add the remaining salt and the pepper.
Gradually whisk in the oil in a thin, steady stream until a medium-thick vinaigrette forms.
Halve the potatoes and combine with the chicken. Add the capers and vinaigrette, tossing to combine. Fold in the watercress just before serving