"In some countries legislation would now be harder to legally enforce that a passenger is vaccinated (what is that 2 dose 3 dose 4 dose - some countries do not offer these extra doses to everyone and you can’t buy them). "
I've had three doses and still got Covid. I expect that I will get it again. If I was very fearful of getting it again, I would cancel all future cruises and stay home. You can't rely on either your own vaccination, the vaccination of others or even natural immunity to prevent you from getting Covid.