I know nearly nothing about sea travel except for a few experiences I've had in the past on cruise ships. But cruise ships are hardly adventurous (I don't like them at all actually). However, traveling by cargo/freight ship does sound like a lot of fun and Porter and Sara Fox ended up getting an opportunity to do just that along the St. Lawrence River (or seaway) which leads straight into Lake Ontario and then into the rest of the Great Lakes. The ship they were on was called the Algoma Equinox.
It turns out that it's a pretty regular thing for cargo ships to take on passengers on most trips. The cost isn't too crazy and you get to get off wherever the ship docks.
It was 5 p.m. on a warm June day. The sun was still high overhead and the air smelled like river water and algae. Fluorescent lights gave the interior of the ship a pale blue hue. The halls were timeless in a way that any steel room, like a prison cell, is timeless. My cabin was on the third floor, starboard side. It was surprisingly large. The queen-size bed could have been transplanted from a Comfort Inn. The separate sitting area had a chipboard desk and mini-fridge, and there was an en suite bathroom by the foot of the bed. The walls were covered with white plastic panels. The curtains were a kind of shiny plastic I had never seen before. Behind them, two oversized portholes looked out on a constantly moving scene.
And if you're wondering about pricing, well, it's laid out in the article as well.
Many shipping companies like CMA-CGM, Canadian Maritime, Rickmers-Linie and Grimaldi Lines offer passenger cabins on certain routes. Prices average around $100 a day for trips to most major international ports. Specialty travel agencies like Maris and A la Carte Freighter Travel book trans-Atlantic and around-the-world trips, and others like ZIM Integrated Shipping Services take applications for artist residencies on their ships.