You make an excellent point. The problem is that you never know when Patient 0 has already touched foot on land and the epidemic is underway. Living in fear of such would mean no one would sign up for another cruise.
I, on the other hand, have no such fear because my fear is being mortally afraid of being on any substance that I can’t walk away from (like water and air), so will not be signing up for any cruises (or airplane trips) in the near (or far distant) future.
I believe you and I could be friends. If there's not ground under my feet, it's a bad day.