CDC recommends against going on Cruises

Air exchange and filtration systems on jet airplanes are extremely good. The air exchange on a plane happens very quickly.

Cruise ships not so much. They're fairly basic systems like a home or office would have. They aren't continually recycling, filtering and exchanging the air like pressurized plane does. They cycle on and off as temperature demands and they don't move the required volume of air.

Ever notice how farts on planes dissipate faster than ones in a room?

Humidity could be a factor, unsure though. There are many more interactions on a cruise, social, physical, etc. You're picking up booze at one counter, then going to the food buffet next, then touching all the same rails on the boat as you walk to the pool area where people won't be masked when swimming, then going to a show, then the dinner hall, etc. Just more potential exposure points.

How do you know, there are no reunions of plane travelers 2 weeks after the fact. Its obvious on cruise ships when you got those same people in one spot for longer periods of time.

You don’t spend an incubation period on a plane so there’s a lot less chance of your infection being attributed to the flight.

Cruises usually last much longer than flights.

Even if you have no prior immunity from vaccination or previous infection, your immune system has some capacity to withstand the virus. If you have some immunity from vaccination or previous infection your immune system is even better at withstanding the virus.

When you experience an opportunity to get infected with SARS-CoV-2, whether or not you actually get infected depends on whether or not you get enough viruses in a sufficiently short time to exceed your immune system's capacity to stop them before they can replicate enough to make you sick or infectious.