The biggest difference of all is that not everyone gets an actual bed, or wants one. On a ship with a capacity of 600 people — about 400 were aboard on my mid-July trip — there were berths for only about 300, with most of those inside four-bed staterooms. That fact creates the crucial dynamic of everything on board — all revolving around the question of where to sleep and, by extension, which of the ship’s subcultures to join.

Some uncruisers stake out the front observation lounges, rolling out sleeping bags or cots between the rows of seats by night, keeping the tables as base camps by day for cards, reading and meals they brought from home or bought on board. Others claim the reclining deck chairs, which, over the course of several days, can become entwined neighborhoods of books, guitars and backpacks. Others sleep in the movie theater.

I threw in my lot with the tenters.

Through five decades of Alaska Marine Highway tradition — the ferry line was founded in 1963, only four years after Alaskan statehood — the open expanse on two aft decks has gained a kind of mystique among many Alaskans I met on board and in my later travels. Tenting north, they said, was more than just a place to rough it under the stars or travel on the cheap, but rather a kind of portal between the fusty old rules of the lower 48 and the unbound sense of space and personal freedom that has been Alaska’s magnet for generations.

In the 1970s, for example, tent city was the ferry’s party headquarters, where oil workers, fishermen and loggers like Steve Goldsmith gathered to celebrate their way north.

“They’d say, ‘We’re in Alaskan waters, boys, light up,’” said Mr. Goldsmith, a 59-year-old tent camper, recalling the shipboard announcement that the ferry had cleared Canadian waters and fussy marijuana laws.

Things are quieter now, with more families and older travelers like Mr. Goldsmith, who left the timbering life years ago. Open consumption of alcohol outside the bar and dining room is banned, or so a sign advises. What has not changed, though, signifying another major difference from the traditional cruise, is that almost everyone on the Columbia was actually headed somewhere. Like a commuter ferry, though on an Alaska-size scale, the trip for many of the travelers was a means, not an end in itself.