Place the potatoes and water in a saucepan; bring to a boil over medium-high heat. Cover and cook until tender; drain and set aside., In the same pan, saute onion in butter until tender. Stir in flour until blended. Gradually stir in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; add the potatoes, salt, celery salt and pepper. Cook for 2-3 minutes or until heated through. Sprinkle with parsley and paprika.