The master of a vessel is liable to third persons for the acts or negligence of persons employed in its navigation, whether appointed by him or not, to the same extent as the owner of the vessel; but neither the owner nor the master of a vessel is responsible for the negligence of a pilot whom he is bound by law to employ, unless an option is allowed between pilots, some of whom are competent, or unless he is required only to pay compensation to the pilot, but is not required to use his services.