Don’t you think that cruise ships would filter the air supply before blowing it into rooms?
In any case, even if the passenger in the adjacent cabin is coughing and sneezing, any contaminated air is going out, it is not being mixed with the fresh air coming in.
https://www.cdc.gov/norovirus/about/prevention.html
The CDC’s suggestions for norovirus will also work against coronavirus.
Sure, but the efficiency of the filters drops as the size of any passing particles is reduced. And as for mixing in air, I don't know if they do partial recirculation (as on a plane) to conserve energy in air conditioning/heating, or whether the air is all taken from the outside and cycled through the rooms once. I could imagine either solution, but that's not knowledge on my part.