Here's my answer to your question.
It's as safe as traveling on a plane, train, bus, or any other mode of mass transit.
It's as safe as attending any sporting even, concert, political rally, or any other heavily attended event.
It's as safe as going shopping at a grocery or department store, even around the holidays.
It's as safe as going out to breakfast, lunch, or dinner at a popular restaurant.
It's as safe as going to a church, temple or mosque for religious services.
It's as safe as going to a bar, nightclub, or movie theatre for an evening of entertainment.
It's as safe as going to a party or a large family gathering.
IMHO it's SAFER than going to a hospital or clinic. That's where sick people, germs and viruses are concentrated.
Anyone can get Covid by attending any of the functions or events I have listed above. So if one regularly participates in any or all of them, they should have no issues going on a cruise.