The people saying they're the only way for people who can't fly/are disabled to travel- that's definitely not true.
I had a close friend who wouldn't fly because of fear, and I now know a few people who won't fly for environmental reasons. They have all tended to go for trains or ferries, have holidayed across Europe doing so: most cruises that go any further than the med people tend to have to take a flight at one end or the other.
Disabled access is something that should be improved in general, but I can't see how the cruise part helps offset the challenges in cities that aren't accessible: the accommodation could be anywhere, and when you're exploring the city you're in the same situation as if you got there by other means.
Passenger ferries are actually much lower emissions than cruise ships: we ended up driving to Switzerland from Ireland a few years ago for slightly complicated reasons and out of interest I compared the carbon to if we flew, assuming that it would be worse (long ferry, lots of driving) and in reality it was something like half. But cruise ships have so much space given to accommodation, amenities, etc etc it's incompatible.