A boat licensed to carry passengers may also carry cargo, every ten cubic feet of cargo being considered as equivalent to one passenger, but the total of the number of passengers so calculated and the passengers actually carried shall not exceed the number of passengers which the boat is licensed to carry. If no passengers are carried, a boat licensed to carry passengers may carry cargo, subject only to the limit of free board of nine inches provided for in rule 14. No such vessel shall carry cargo such as fish, manure, bones, skins or other articles which in the opinion of the canal officer is offensive or dangerous.