I looked that one up. They use a real roll on/roll off ferry, self propelled, like Europe and New Zealand, and probably other places. I didn't know we had any of those in railroad ocean service, although Washington, BC, and Alaska have ocean-going automobile ferries.
South Florida to Puerto Rico wouldn't be much farther. It would be interesting to see some PR cars up here, if they still have a railroad. They used to.
On the other hand, I can see why they don't use something similar for the Alaska service. I doubt that anyone ever tried to send 100+ cars to Alaska at one time...