EDIT: Eh, never mind. I don't want to get a thread locked because we talk about Covid-19 too much.
I'll just say that a fully vaccinated family can still get rather sick. They likely won't require hospitalization, but they will likely require antiviral medicine and/or steroids. Hopefully, the cruise ships start packing enough medicine to handle the increased caseload.
Anyway, at least on Disney's ships, they aren't quarantining everyone that tests positive and their contacts. They're only quarantining the person that tests positive. Everyone else in the room is advised to wear masks, but otherwise they're free to enjoy the rest of the cruise. That doesn't apply to someone that tests positive before the cruise, of course. Then, sure, the whole group is denied boarding, but not quarantined.