I think a lot of you are unaware of how much care and trouble cruise lines go to so that people stay healthy. The main reason for illness on a cruise ship is the filthy bastard passengers who won't wash their hands, but these people are everywhere.......hotels......campsites....airplanes... As on all holidays, you need to pay attention to detail for yourself. So, wash your hands, use the stairs not the lifts, use a napkin to pick up serving spoons if you want to use the buffet, use the toilet facilities in your cabin not the public ones.
I have several health issues, but I feel very safe on a cruise BECAUSE I see so much cleaning going on. And if it is the air con you are worried about, if that was the route for most infections, then you wouldn't be any safer than anywhere else that has air con...offices, shops, hotels, GP surgeries, etc.
If it is the quarantine failure on the Princess cruise ship in Japan that has coloured your view of cruising, I do believe it was a mistake to keep people in quarantine on there, but not because I think the air con was passing it round. It was much more likely to be passed via crew members taking food to passengers kept in their cabins. Food itself was probably OK, but the trays, plates, cutlery - probably not so much. I hear that the crew worked very hard, and I'm sure they did their best, but they were not trained in proper quarantine protocol. There was also the fact that passengers were allowed on deck for short periods, so touching handrails, lift buttons etc which were probably cleaned in between batches of passengers being allowed on deck, but if a passenger incubating Covid-19 was in your batch and you touched a surface he/she touched, then that would be a route of transmission.