I just got off a QM2 transatlantic crossing(which is about as captive/boring as cruises can get?) Friday. I went on a lark, I have never “cruised”(and had a pretty negative view of it - all people do is eat and drink etc)but had an itch to cross the ocean on a ship in old timey titanic type way. I’m also ~30(which might as well be 16 on a cunard ship - very very old crowd) and went alone. I expected this was mostly a bucket list type thing people did once, or a select coterie of people who both need to travel and cannot fly.
My most surprising takeaway was that almost everyone had done it before, a lot of people many many times. It was legitimately rare to meet a first timer, even among the relatively young group I fell into.
The boring bit is more subjective, it was more fun than I expected. I can see why people love it. “enforced” relaxation, no stops, peaceful, easy, etc.