We are not allowed to speculate on these boards, but looking back to March 2020 , that is how the whole Diamond Princess mess started.
That's true, but everyone was required to stay on board the Diamond Princess and almost nothing was known about transmission at that time, so it spread very quickly among the 'quarantined' ship. With all the protocols in place now, it is a much less likely scenario.
The Breakaway docked in New Orleans and everyone on board was tested and left the ship. Everyone that tested positive was asymptomatic (probably owing to it being a 100% vaccinated sailing). The crew that tested positive have been isolated. The positive guests were given either quarantine accommodations on land if they had been scheduled to fly home (and now couldn't), or were allowed to return home on their own if they had driven to the port.
The Breakaway left again that same day with brand new guests on it's next scheduled sailing. There was one case of Omicron detected, but it was in a crew member from South Africa, who had been in complete isolation (per protocols) since he joined the ship. That
could trigger the Breakaway to be classified as Red, simply because Omicron is a variant of concern, but so far the CDC doesn't seem to think red is warranted.
The cruise lines aren't required to divulge the number of positive cases to the public. The only reason we know about the Breakaway is that Louisiana/Port of New Orleans
does require it to be alerted to the number of cases and that information was public. 17 total cases is too small a percentage for the CDC to mark them as Red, but enough to trigger the Yellow and an investigation. Which kinda means that any ship listed as yellow could have the same number of positive cases on board, we just wouldn't know it. Yellow is triggered when the cases exceed .1% passengers (or one in a thousand), or a single crew member. There's really no way to tell if a ship is Yellow because of a single crew member, or a tenth of a percent of passengers...
I'm not going to be too worried about anything less than Red