Quite a few comments from "I know I wouldn't like it" folk. We know a chap who said the same, for years. Finally his wife got him to try a cruise -- now he cannot wait to go again!
We went on a few - when we were both working! -- and there's LOTS to do, but you do NOT have to join in if you don't want to.
Plus of course the ship stops at ports most days (that's the whole point?!) unless you're going across the Atlantic or Pacific.
There's a cabaret/show most nights on most ships, plus a bar or two will likely have a singer or pianist for an hour or two.
As for the food, yes, you could overeat, I recall finding myself in a deckchair one afternoon next to one of the entertainers. I asked him how he avoided eating millions of calories a week, he said HIS routine was to only eat lunch (plus teas and coffees I think).
I recall on one ship we used to hit the buffet, after a late-evening stroll, we could get a tea or a fancy coffee without a big crowd. The boys and girls of the dancing troupe used to get there about 11pm, (after their second evening show), they struggled to eat much in the day what with rehearsals then shows, plus helping with daytime entertainments.
We also used to occasionally get up on deck for a sunrise moment, and you'd find regular joggers and power-walkers trundling round whichever deck was marked out for them!
I also recall taking some crochet 'kit', and I guarantee if you're like that, out on deck somewhere, you'll soon draw some new friends wanting instruction or the pattern?!
There are specialist cruises for Bridge players - and there's plenty to do for their stranded bridge-widow/widower if that's the case! Others for gardening themes, wine enthusiasts, all sorts. If you're new to the cruising idea, have a look at one of the specialist sites, 'IGLU' is good, Bolsover Cruise Club, lots of them.
We did meet one or two people who cruised year-long, though I think they swapped ship somewhere?
BUT -- though we like cruising (especially on a smaller ship, a floating city is NOT for us!), we couldn't do it year-round.
If you REALLY like, say, theatre, would you go EVERY night, and EVERY week?