Actually this leads to an interesting point..as far as I know cargo ships are not automated sooner. It seems that it's just as open as the sky...potentially the sea is much less forgiving?
I would mainly say less standardized and less regulated. Big vessels already require pilots in and out of ports because the crew isn't trusted. Then essentially every single ship is a custom build with all the years of problems to iron out that comes with.
With less regulated you don't have a required AIS on every canoe or vessel, meaning you need to react on visual and radar input according to the colregs. In the US every aircraft flying already have ADS-B.
So with some worldwide regulation and a modular system able to interface and work with different equipment and sizes to create the economics of scale you have something working in the not too far future.
Though, this still requires the bridge and interfacing systems and sensors to be stable enough. Meaning no vessel today is good enough.
Compared to aviation it is much more hands on based on tribal knowledge. Like on a ship I work on "the remote for the autopilot tends to quit working every 3 months, just reset the whole autopilot, by pulling the breaker, if it does." This information stays on the boat and never reaches the manufacturer, instead of fixing what is probably some overflow happening. And the fix would be replacing the unit instead of deeper troubleshooting. Seen it happen many times.