Ombudsman Jim Martin said the transport agency must look at measures to reduce weather-related cancellations and improve reliability on the Dunoon-Gourock ferry route.
The recommendation followed a complaint by campaigners over the awarding of a six-year contract to run the service to CalMac's Argyll Ferries, which began on June 30 this year. The service has been run on a passenger-only basis ever since, ending the previous CalMac car ferry operation.
Four separate complaints about the tendering process were brought by Dunoon-Gourock Ferry Action Group, but none of these were upheld by the ombudsman.
A spokesman for the agency said it will continue to work with the group to find a solution regarding a vehicle and passenger service.