It's hard to put a figure on % certainty. It just seems completely unreasonable that multiple cruise ships would have outbreaks of the same virus over a short period of time...unless there is a common means of infection. Norwalk is spread by poor sanitation e.g. people not washing their hands after using the restroom. Smallpox is spread by face to face contact, contact with contaminated bedding and contaminated dishes/silverware. The use of a food service to "inoculate" the vessel is dirt simple. People are a lot more trusting of silverware than restroom doors.
That's possible, 99% of the crews on these ships are foreign.
But I don't rule out a psychosomatic cause, perhaps one or two real outbreaks of fle, followed by a hyper suggestibility on the rest.
Like when an entire high school suffers a "mystery ailment" of mass fainting or mass nausea, and it's purely in the head.