Years ago a friend checked with a Cruise Ship line along this same concept.
It was considerably cheaper than any retirement home or nursing home with meals all day.
The problem with that plan is that when you become seriously ill or injured, the Cruise ship staff will unceremoniously dump you off the ship at the very next port and it will be up to you on how you get treatment and get back to the US.
You can have insurance set aside for that eventuality, but it can get very expensive, depending on your condition and the limitations built into the insurance. Also, the onboard staff will not be your nurse if you have mobility issues that requires regular assistance in your cabin or elsewhere onboard.