Before we get to autonomous ships in international trade in a decade or two there are a lot of development ongoing.
Although shipping is not covered in the now famous Paris Accord it is a main concern within IMO.
Norway wants to do it’s part to ensure that emission targets are met by taking the lead within IMO on this aspect. (Since USA has abdicated from it’s leadership role): http://en.portnews.ru/news/240048/
Japanese shipbuilders and DNV-GL is also doing their part to develop ECO ships based on improvement of existing technologies, while we wait for hydrogen power to become available: http://en.portnews.ru/news/240048/
For short sea trade the battery technology is making strides: http://nor-shipping.com/press-release-future-fjords-revealed-ground-breaking-zero-emission-passenger-vessel/
An immigrant American of South African origin is a prominent supporter of the development of improved batteries as power for ships as well as vehicles and other application.