I’ve actually heard of others doing the same thing, and it’s also easy to forget the disadvantages that come with it too.

I wasn't suggesting it has no downsides, but rather, in her case, the downsides are outweighed by the advantanges.

You don’t get very much space to yourself and no real permanent changes allowed.

These are identical conditions to many retirement communities which is another frequent choice of those in their 70s like she is.

No choice in much of anything available to you. No realistic option to own a car, so anything inland is much less accessible.

I'm not see this one as applicable here. I don't know how long they stay in port, but if you're only spending $2500 per person per month for housing and food you likely have a good chunk of money left over. Their house moves with them to other countries. I'm not sure why they'd what the expense or headache of car ownership. Also nothing would prevent them from renting a car or taking public transportation to get deeper into country they're in.

Not a bad life by a long shot, but a lot less autonomy and personal choice in return.

Certainly not for everyone. I don't think I'd want to do this for the long term, but the tradeoff of very cheap travel and living expenses at a time when otherwise advanced age would prevent travel can certainly be appealing.

No realistic option to own a car,

that would be a huge positive