I’ve known a couple of people who do/did these trips. Both were retired Ph.Ds in their field:
A retired Art History professor from Georgetown (specialty: Silk Road) does them several times a year for the Smithsonian and in fact is on one right now. I don’t think all of his gigs involve cruises per se; they are at least sometimes land-based. He goes to Turkey, Morocco, Spain, Portugal, Sicily, and even did one in Vietnam a while back.
The Smithsonian runs a tight ship, so to speak. Their paying participants are asked to rank the quality of his lectures every time he goes out. When his scores dropped below a certain level (in Vietnam; I assume because he’s not as familiar with the material, and he said there was one guest who was a PITA), he was put on probation! But he’s fine now.
His biggest complaint about the work, which he mostly loves, is that he is expected to mingle with all the travelers. He’s an introvert and so that can be a challenge.
A Harvard professor, now deceased. I forget what his specialty was but I am sure he was a fantastic lecturer, as he had a wonderful sense of humor and a booming voice. I also forget who his employer was, but I assume Harvard University. He did primarily do cruises, I believe. His feelings about it were mixed - he said some of the guests could be demanding assholes.
We get brochures trying to tempt us into taking such “intellectual” cruises all the time, from our various alma maters. They have included wonderful sounding adventures to the Galapagos and Greenland, among other exotic locales. The lecturers are always current or retired professors from whichever alma mater is involved.
So, based on the above, I tend to assume that many of these trips are organized not by the cruise ship companies, but by universities or museums, who provide two components: the academic expertise, and access to a mailing list of alums/members who may sign up. I imagine they work with cruise ships/travel agencies to take care of the other details.
That’s all speculation on my part, though. Still, I doubt the cruise ship companies are the ones to talk to.
ETA:
Nope, they get well paid. That’s why my art historian friend does it.