What about background checks on paying passengers?
They can check background on employees day in and day out but that does not prevent a passenger from doing the evil deed. I suspect a vial of something brought aboard by a passenger, sprinkled onto the buffets, on the ice, or wherever. Could even be sprinkled on linens as they walk by.
I agree this is terrorism and is intent on harming the cruise lines economically. This could easily be done with a deadly virus also. Wonder if a message is being sent?