Hi brenners8080
You do not have to be worried about the capacity on the boat, as the main season for boat trips here in Switzerland is during the summer, between May and October. In fact, in winter, there are only a few boat services per day; I suggest you are checking the timetable carefully: https://www.bls.ch/en/fahren
In winter, boat cruises are famous at night, the boat companies usually offer a Fondue cruise, where they serve Fondue or have other dinner themes.
The Swiss Travel Pass is your ticket, you are fine to get on board with it. The ticket inspector will pass by at some time. In case the boat is well booked, which I doubt, you could still buy an upgrade on board and go to the upper deck where is the 1st class.
Please let us know should need more help.
I wish you a pleasant time in Switzerland.
Best regards,
Roland