Civilian GPS (as in, the DoD’s Navstar) alone isn’t accurate enough to actually place you on a specific road. To compensate, auto navigation systems will snap you to the nearest road parallel to your current heading. Tesla likely didn’t consider the edge case of ferries and other off road situations in their mapping software and you hit some corner case bug.

This used to be true, but the DoD turned off "selective availability" (the intentional degrading of the civilian signal) back in 2000, and the current generation of satellites do not have the capability (https://www.gps.gov/selective-availability). What they do have though is a separate, encrypted, military broadcast that can be used for those purposes (I think the plan in those situations is to turn off civilian GPS entirely - but I think that there's no reason for that, now, due to the other navigation systems like GLONASS).