The only way to find out is to go , just try a 7 day one would be my advice you will soon discover if you like it or not,as for trips entirely up to the individual, personally I don't like travelling around on mini buses/coaches there's plenty of experience on board the ship to offer advice & there's plenty of things on offer if you just want to stay on board . Our first one this year was only £1200 for both of us including flights, terrific value for money.
I'm the same with coaches and mini buses - OK if it's just for 20 minutes or so, but tiresome if it's 2hrs each way. And as I said, ports a grim places*, so generally nothing to look around within walking distance.
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Most cruise lines organise Shuttle Buses to the towns so you just walk off the ship, get on a bus and fifteen/twenty minutes later you get off at the drop off points. Some Ports you can walk to the town but for sightseeing such as ruins/volcanos etc you’re probably better off on an organised excursion from the ship. If you are on a ship organised excursion you know that the ship will not leave port without you if the tour coach is late returning, as it’s their tour so their responsibility. Also, some people like to stay on board at Ports as there will be less people onboard around the pools etc.
What to do onboard is down to the individual preference, I mean, what do people normally do?. We started on RCCL (Royal Caribbean Cruise Line) and they have theatres, ice skating shows, pools, flo-rider surfing, rock climbing, jacuzzi’s, games rooms, gymnasiums, casinos and various clubs to entertain. P & O have most of that, apart from the ice skating, but is less glitzy. We started cruising years ago and I honestly thought my Mrs would hate it (it was her choice not mine), as she doesn’t even like sitting on a beach for more than half an hour, but she loves it.
We always, always, always book a balcony room and if we couldn’t get one we wouldn’t book. As you say, it’s not for everyone, but I certainly wouldn’t class it as a floating hotel.
The Embarkation and Disembarkation (sail only) is so much easier when compared to flying as once you are on board your holiday has started.