"relatives last saw the woman Saturday night, but did not report her missing until Monday"
Not really "two or three days."
There is a lot to do on a cruise and the Imagination is pretty good-sized.
It VERY possible to not see someone for a day or two if your schedules aee a few hours off.
If the family thought she was off doing something else or didn't want to disturb her while they thought she was sleeping this is a very probable scenarion.
You're being pretty hard on folk, mtb, without knowing more details.