The ferryman has gotten very busy over the years, now that there are literally billions of people living and dying on Earth. Many of these souls can't afford passage. To solve both of these issues, he has created an entire flotilla of oar ships to ferry souls across to the underworld, allowing a number of souls to work off their debt by literally becoming part of the craft themselves. These souls are the materials from which the ferries are made, and by which they are powered. This doesn't show up too well in my image yet because I haven't started putting in any details at all, but they will be made of bodies, bones, etc.
This rough sketch is to get the basic perspective and general forms in place.