Once again, I’ll concede a narrow point.
Used cargo ships (especially OLD used cargo ships) don’t make economic sense if you’re trying to start up a cargo ship line competing with the established shippers in the container movement market.
Used cruise liners (especially OLD cruise liners) don’t make economic sense if you’re trying to start up a cruise ship line competing with the established Cruise Lines in the up-scale mass tourism market.
Despite some opinions that "Marine technology as we know it is not capeable to put competitive real estate square meters on the ocean. It was not developed for housing purpose and it can not be adapted to housing purpose. (its cost is to high). " I believe there is a way. And that the way a used ship makes sense is in becoming “ocean real estate”, not “ocean transportation”, unless you can find and operate in a niche that competition has neglected, i.e. small ports, local runs with high value, small volume or requiring specialized handling. “Ocean Real Estate” to my mind includes “processing facilities”. If you’re producing a product on your ship, you’re not merely transportation any more and that changes the equation.
One sector that seems viable on the face of it, is small-batch (in the global context) mobile fuels processing. Pyrolitic re-processing of waste plastics into fuel, bio-diesel, and bio-gas production from waste agricultural products in areas where local economies and foreign investment have neglected the niche. A key point would be economically moving from location to location, or else potential profits would be eaten up by the movement. I think the essential technology for that economical movement is supplemental or primary wind power retro-fit with one of a couple designs- wingsails, Flettner rotor sails, or skysails (para-sails).
In addition to processing of fuels, there is the opportunity for value-added services to local economies- agricultural outreach and consultation to improve bio-fuels’ input stocks, energy consulting for renewables and green energy, including solar and wind power, as well as installation services of same.