All gangplanks, slings and other appliances running from such watercraft to such dock or wharf whereby rats might pass from such watercraft to such dock or wharf, shall be withdrawn when not in actual use in loading or unloading freight or passengers, or shall be suitably guarded to prevent the passage of rats; provided, that watercraft plying between ports in the State of Washington only need not comply with the requirements of this Section when at such dock or wharf for less than one hour between the hours of 7 a.m. and 6 p.m.