In the last comment thread, commenter papa noted a very natural first guess: that the title was a riff on the musical Showboat, and that the post topic would thus be space liners (rather than military showboating).
Thread drift duly followed, so that the rest of last thread can usefully be read as 'pre-comments' on this post.
If you do a Google Images search on battlecruiser, images of space warcraft handily predominate over, you know, actual seagoing battlecruisers. Do the same for space liner and you get a strange grab bag of images - the first row on my screen includes two aerospace vehicles (one from the film 2001), a very stylized deep space craft (from a Maldives stamp), a car, a bicycle, and a pullcart for garbage cans. Spaceliner as a single word brings up mostly bicycles, plus a fair number of buses.
This little search adventure confirmed something I already knew from my traffic stats: We geeks are a rather bloodthirsty lot, at least in imagination.
It also showed me something a bit disappointing that I didn't know. Space liners do not seem very well established as a trope. Even the Evil Website has only one reference (relating to the film The Fifth Element). Yet a future of large scale space travel, interplanetary or interstellar, might reasonably be expected to include space liners, so they are worth considering here.
First, some definition of terms, then a bit of future-history speculation. The minority of Google images showing spacecraft of any sort are dominated by aerospace planes, like 2001's Pan Am shuttle. (Melancholy note that the commercial tie-in was to a now-defunct airline.)
Operators of aerospace liners have a perfect right to leave off the 'aero-' part, but for this discussion I have in mind 'real' space liners that operate entirely in space, especially those making long trips through deep space, interplanetary or interstellar. And I am concerned here with spacecraft intended for travel, from one planet (station, whatever) to another, not the spacegoing equivalent of cruise ships.
(The spacegoing counterpart of the cruise ship is - at least for a long time to come - the orbital hotel. It has simpler operating and emergency procedures, and offers a scenic view for the whole time, not just near departure and arrival.)
If we have anything more than minimal human travel into deep space, we will presumably have the space liner's more utilitarian counterpart, the interplanetary transport. (Some technology assumptions, notably cycler stations, more or less eliminate this role, but I'm somewhat doubtful of these approaches.)
Commenter Tony made a suggestion last thread that early transports, at least, may have only a minimal dedicated crew - the passenger manifest providing needed most of the needed skillsets as well as the scut labor. After all, these early passengers will be trained specialists on assignment to outpost destinations, whatever the institutional details.
I tend to agree. Indeed, as I noted in comments, early passenger transports may have no permanently assigned crew. If they are lightly shielded to save penalty mass, a couple of round trips might rack up a substantial fraction of lifetime radiation exposure, making ship's crew a non-viable career path. And the people being transported to and from deep space outposts reasonably include propulsion techs, along with mission controllers who can function as pilots.
Somewhere along the time line, however, VIPs will muscle or bribe their way onto the passenger manifests. The first of these will make no difference in terms of accommodations - like today's pioneer orbital tourists, they will bunk and eat alongside working crew/passengers. At some later point, VIP passengers, whether bureaucratic or paying, will become numerous enough that First Class accommodations will be provided for them.
At this point the true space liner begins to take form, even if its chrysalis is just a corridor or two within the hab of an otherwise-utilitarian transport. As suggested in the last thread, even the transports will be relatively comfortable, because their travel times are in weeks or months. First Class accommodations may offer slightly larger cabins, or roomettes instead of bunkrooms, but their most distinctive - and expensive - feature will be stewards and other hotel staff.
If bases and outposts do evolve into incipient colonies (by no means a given!), travel will become more commercialized and purpose-built space liners will evolve. These may still have some equivalent of steerage class for passengers who do much of the work of maintaining their hab spaces - at least the basics like making up their own bunks and providing galley services. But they will also have First Class and intermediate level accommodations.
Yes, there is also the possibility of that old SF standby, Cold Sleep, or some other means of snoozing for most of the passage. But the more we learn about humans biologically the more complicated we turn out to be, and I don't think we can count on this.
In any case, by this point we are well beyond the 'Plausible Midfuture.' Will the liners, or at least their First Class sections, have spacious atrium areas with gardens and the like, to provide the illusion of not traveling through space while confined in a can? If your setting is operatic enough to have full-blown space liners, I wouldn't rule this out - though a lot of the spaciousness may be artful illusion.
The image at top is one I made several years ago, representing the liner-freighter torchship Silk Road. Due to the (improbably!) high acceleration no spin is needed, and the passenger pods are shamelessly based on rail passenger cars - even to the 'baggage car' clamped onto the passenger section.
The image below has nothing to do with space liners. It is a Blackpool (UK) 'Boat' - so named for obvious reasons. It is about as cool as streetcars get, and this one sometimes runs in regular service on the SF MUNI's F Market line.