Ships were used in the ancient world -- as they generally are now -- to carry freight from nation to nation for trade. The various valuable items in the cargo represent desires for specific good things and individual ideas about how to accomplish good things. The ships, then, represent containers for those good desires and true ideas -- they represent doctrinal systems, large collections of good desires and true thoughts woven into a whole and working as one. For instance, if we believe that we need to turn to the Bible to learn about right and wrong, need to compel ourselves to do what's right even if we don't want to, and need to trust that the Lord will help us come to love doing what's right, that would be a doctrinal system -- a bunch of ideas and desires for good all working together. Specific ideas -- we should read the Bible, we should pray, we can trust that the Lord is helping us -- are like cargo that is carried along by that larger doctrine.