Have you ever tried to fly a quadcopter drone manually? It's really hard; I could barely get one to hover, after hours of practice. Yet AI quadcopters that can fly in ornate formations and execute complex maneuvers are well known [1]. Don't discount what AI can or can't do just because it's hard for humans.

Other advantages in ferries' favor; unlike most streets, the docks for autonomous ferries could be designed around the AI's capabilities. Also, ferries don't travel at 70 MPH. :)

[1] https://www.youtube.com/watch?v=YQIMGV5vtd4 (6-year-old video!)