Cargo ships are largely autonomous once underway. They have a small crew of 8-12 members who largely do maintenance, watch, and cook. Then, near shore it's all hands as they prepare the ship for docking.
I suppose the "cooking" part can be eliminated once you don't need maintainance and watch :-)
"Watch" is an interesting (and solvable?) challenge. Understanding the sea environment (visually, sonar etc) from the local (ship's) perspective can complement e.g satellite imagery. But there is also e.g. watching against piracy
Maintainance seems the toughest. Unless the engineering is perfected to the point that it is only required at ports.
You will still need mechanics to solve things aboard no matter the navigational automation, which allready are on autopilot on open sea.