Ultimately a ship is a system which is broadly similar to that of an aircraft. Everything's scaled up, but the basics - a means of propulsion, a control system, a navigation system - are very similar. We can make autonomous systems which are exceedingly reliable (satellites make it to 15, 20 years in orbit without failure or maintenence on a regular basis) and we've been operating large ships for long enough to have a good idea of where maintenence is required.

A ship's engine room may be larger than the engine of an aircraft, but the systems aren't more complicated. Proper maintenence procedures and redundancy where needed are already used in other systems. Why not ships?