Yes, you are funneled into one of the storylines that will determine your missions on Batuu and your experiences the second day when you return. But no, it isn't wholly random - if you show up in a shirt that says "Bounty Hunter Services," you're probably going to be sent into the Rogue storyline (which I was).
Just got back from the 4/14 cruise and had an absolute blast with the whole experience. I'd be surprised if there were any mechanism that took into account your apparel absent other input. There are a couple general Datapad questions early on with 4 possible responses (the way they sync up with the paths is transparent). Combined with that, you have the ability to approach the various face characters early in the first day and be physically scanned into particular missions that will help determine your path. My spouse and I combined to cover most of the Rebellion and Scoundrel stuff, working closely with the corresponding face characters. I could not be more impressed with the cast for the entirety -- they learned our names and how to recognize if we belonged to their faction in such a short amount of time.
If you completely ignore the Datapad on Day 1 and don't engage with the story moments, I suspect your Day 2 itinerary would look pretty empty. There certainly was a non-zero number of people just wandering around the ship with drinks in hand at all hours, so maybe that's what those people did. Not my idea of money well spent, but to each their own.
I should also say that the Resistance/First Order split on our cruise was really pretty good. The First Order did an exceptionally good job at recruiting from the large pool of children on board, to the point that jokes were made about that in the final sequence.
Others talk about being picked up from their hotel with Ubers or private cars, which was never advertised (AFAIK) as a service.
They got us a taxi to head to the All-Star for the rest of our stay at WDW and covered the cost. Wholly unexpected, but very appreciated.