It's weird that this isn't already the case. We do it with airplanes, and surely that's much harder simply by virtue of needing to consider three dimensions. I can't imagine it would be difficult to automate steering a ship across the open ocean. You could build something basic and just make the captain handle all the iffy stuff.
Autopilot has existed for decades now and works really well, but it doesn't replace a crew, be it only for everything unrelated to navigation, crossing the path of other vessels, etc.