NSSLGlobal has written a white paper on this subject, it can be downloaded from this sight;
http://www.nsslglobal.com/droneshipping?=twitter
The conclusions are that it is unlikely that there will ever be truly autonomous ships, and drone ships for low value bulk cargos may start being built sometime in the 2030’s. The report further states that dangerous cargos will probably never be carried on unmanned ships.