My first cruise was a 30 day leg of a round the world voyage on P&O Aurora. Hong Kong to Southampton. We stopped at Bangkok, Singapore, Kuala Lumpur, Penang. Kerala, Mumbai, Sharm el Sheik, Port Suez, Athens, Barcelona, and (finally) Southampton. Only a day in each port, and plenty of sea days, but there was exploring enough.
Frankly, I prefer this over flying, if I have the time, and I can find a ship going in the right direction. I don't have to pack and unpack every day, it's relatively cheap, includes all meals, provides an ever-changing view of the world with the chance of whales, and I can spend hours at a time just happily doing nothing.
There are activities for those idle hours. Dance and music classes, art and language, specialised topics, gym, craft, films, talks, shows.
Plus I got to see some reasonably exotic ports. A Suez Canal transit, for all love - not something one gets to do frequently!
Admittedly, not much time to look at a city in depth, but I've returned to Athens and Barcelona since then and spent several days in each. Think of it as a taster.
Drawbacks. It takes time. For a working person, this can be a challenge. There are short cruises of three or four days, but these barely scratch the surface, and are often popular with the drinking classes, by which I mean those who aim for quantity over quality, with the inevitable results.
You are often stuck with the same table companions each night. This can be good or bad, depending on personalities, but whereas a curmudgeonly seatmate is out of your hair in a few hours, putting up with them for days or weeks might strain the resources.
Other passengers might be a different demographic. On the Aurora, it was mostly retired British gentlefolk. Lovely people, but hard to get a conversation going. And frankly, their idea of after dinner shows in the theatre fell short of my ideal. Just who are "The Bachelors"?
Internet can be scarce and expensive. I found myself getting up before dawn to do my stuff, when most of these pensioners were fast abed.
But I loved it. Nothing quite like sitting up of a morning, fresh coffee to hand, watching some foreign shore sliding past.
I have some friends who do a half-dozen cruises a year. Gets into the soul, I guess.