Is that something you can pay to do?
I booked through a specialized travel agent in the EU that has since gone bust, but if you search the web for freighter cruise or cargo ship cruise you should be able to find some other companies that can help out.
For those thinking about doing it, the Asia/Africa routes from Europe are much more interesting. Going across the Atlantic or Pacific is much less interesting. Also in the modern age of container freight you don't have much time at the ports, and often the captains are reluctant to let you get off, because they are on such tight schedule and delays would quickly cost 100s of 1000s of dollars per hour (that will give you an idea about the economic impact of this blockage).
My mother actually travelled on a loose cargo freighter in the 90s. That was a completely different experience because you would often have several days or a week at port.
The cost was about $100 / day, but this was almost 10 years ago, so I guess it went up by now. This isn't really competitive as travel, but if you consider the experience of being on a container ship a holiday in itself, the cost was quite reasonable IMO.
There was one other passenger on board, and we could go anywhere on the ship without an escort except the engine room. I really enjoyed it, but its not for everyone as there is no communication / internet access available so you have to be the sort of person who can entertain yourself.
Also wouldn't go if you get seasick easily. Unlike say cruise ships, the crew isn't too worried about passenger comfort, and wil just plow straight through storms.
