This is going to get interesting. Presuming this CDC injunction holds, then the cruise chips can enforce their rule to allow only vaccinated passengers. What if Covid still breaks out onboard? How to spin it.
No need to spin anything. Vaccines are NOT plastic airtight bubbles around you. If you inhale a load of virus from nearby infected person, there is nothing stopping the virus from establishing residence in your lungs. And you could test positive for covid.
The difference the vaccine makes is......your body is less likely to mount an over-the-top response to the unknown virus multiplying like crazy in your lungs. Because the vaccine would have introduced your immune system of the protein signatures of the virus. This over-the-top response of white cells attacking the virus has resulted in destroying the lung tissue permanently where the virus is residing. In addition to that, the vaccine would have created anti-bodies and those are additional ammo for your body to subdue the virus before you get so sick you end up in the isolation ward of a hospital. Your body will simply create more copies of the anti-bodies and win the fight.
So yes, even on a ship with 100% vaxxed passengers and crew, latent infections could surface and some could test positive for covid. But the good news is most of them will be asymptomatic, and not likely to get seriously sick. No worse than getting catching a cold on the ship. No need to freak out.