The "retire on a cruise ship" deal has been a cute story for a long time. Fun with math, the cost of living there is less than either an apartment or a nursing home. The problem with that is most cruise ships are the same trip over and over. Same ports, same activities, same acts, same food rotation, same everything, over and over and over.
Many cruise ships are in fact floating nursing homes in all but name. When family friends retired and went on a round-the-world cruise, they discovered a fair few of their fellow passengers were repeating the same trip over and over because it was actually somewhat cheaper than being in assisted living.
Depends on what kind of nursing and assistance you need. Cruise ships are hardly "floating nursing homes." Yeah, they make your bed daily and prepare all your meals. But they won't step in if you need assistance getting clothed/bathed/going potty/eating. Medical service is very limited; if you develop anything serious while at sea, count on an expensive airlift to land. You can't get medical equipment serviced onboard; you'll have to hope someone in port can do it and in the limited time you're there. If you arrange to pick up medication or equipment at a port but then that port gets skipped for some reason, you've got problems.
You can pay for someone to travel with you to assist (which can double your cost) but they won't be able to do anything about some of these situations.