I'm a bit skeptical of this. Looking at the numbers for the Symphony of the Seas ship (current largest), the peak power requirements from all propulsion (3 x 20,000KW + 4 x 5,500KW = 82MW) is comparable to the peak power output of the ship (4 x 14,400KW + 2 x 19,200KW = 96MW.)
Of course I doubt the ship rarely, if ever, goes pedal to the metal with all thrusters at once, and I cannot say what it's typical energy generation looks like. So I don't have enough information to say you're wrong, but the numbers I do have make me skeptical.
I'm sure cruise passengers are more wasteful than the average person at home or in an office, but the tremendous amount of power needed to drive a huge lump through the ocean at any reasonable pace dwarfs the pumps in the infinity pool and the blaring music and all the rest. And that's going to have to come from somewhere.
The others were the German Otto Hahn (https://en.wikipedia.org/wiki/Otto_Hahn_(ship)), Japanese Mutsu (https://en.wikipedia.org/wiki/Mutsu_(nuclear_ship), and the Russian Sevmorput (https://en.wikipedia.org/wiki/Sevmorput). There is also a fleet of six Soviet/Russian icebreakers, still in service, the lone regime in which nonmilitary nuclear marine propulsion has proved viable.
Why?
Why is this? Does Carnival have many more ships, or less efficient ships, or run their ships more often?
I actually think companies like Maersk are producing far less pollution than cruise ships because cruise ships are running engines mostly to produce electricity. Only a small portion of what they burn is for propulsion. Imagine the electricity it takes to run an entire city, it would be in the megawatts. For example, Carnival's mega-ships have the capacity for around 6,400 people. The ship is producing electricity for all those people, pools, waterslides, electric go-cart tracks, dance clubs, ~30 restaurants, hospital clinics, and the list goes on. Anything a city would have, the cruise ship has too.
The other big problem with cruise ships is that they produce just as much pollution while standing in port as they do on the ocean (because again they need to run engines to generate a city's worth of power). Some places like Alaska have recently passed laws that require cruise ships to modify their ships to plug into the city's power grid and not to run engines in port. This law is going into effect in 2020 I believe (only for Alaska). But most cruise ships have already complied and are capable of this. I have heard rumors that Italy is considering passing a similar law. The other benefit of this, aside from the environmental effects of it, is that money is now generated for the local city in the form of utility billing to the cruise ships. This stimulates local economies, which traditionally have been hurt by cruise ships more than benefiting from them.
Shipping companies really only run engines for propulsion. They have small crews and accommodations that aren't much different from a large fishing vessel. They can likely generate most their power from their propulsion engines, without the need to run auxiliary generators 24/7.
I am not saying that shipping companies are "clean", but I think they are dwarfed by the pollution created from cruise ships. So it makes sense to target the heavy hitters first.
Plus for an environmental group, it is easier to target a bad guy being an optional service like a cruise. Something people can easily choose not to partake in. It is a lot harder to boycott Maersk. Since almost everything sitting on your desk right now likely sat on a Maersk ship at one point in its existence.