I repeat again: It is not intended to take a traditional ferry, or a ship intended for manned voyages and send it across the ocean. Autonomous ships will be designed built and equipped differently, with lots of redundant systems and minus all the things that is there to keep the crew alive and reasonably comfortable.
It is not a given that propulsion will be by a diesel engine, or electric propulsion driven by diesel generators. If you read the many articles quoted in this thread you will have a good idea of what is likely to be the Future Ships and it looks very little like what you see today.
PS> I don’t think you’ll see ferries and ships carrying passengers being totally without crew, but they may still be technically autonomous, or remote controlled. Cruise ships may have an actor playing the role of Captain for the benefit of the passengers. Somebody have to host the Captain’s Dinner.