Safety?
Two decades I knew a chap who'd been second mate on a container carrier. He'd given this up and gone to work on a nuclear reactor (where I met him) because "it was less dangerous". He told me that, on ships with stopping distances measured in miles, nobody on the bridge was capable of navigating without the onboard technologies. They would "drop the pilot, engage the satnav and put their feet up on the console" (his words).
Given the crowded seas and the reliability of transport automation demonstrated so far ("Windows for warships", Tesla autopilot, 737 MAX), I wonder whether autonomous shipping is the best of ideas. It's a much harder task to stay on course at sea than it is in the air, let alone to follow a road, and preventing collisions requires a lot of attention, forethought and judgement.
The bean counters will love the idea of no salaries for crew, although they might need some emergency standby personnel on board to reboot the computer when it crashes.
Biting the hand that feeds IT
