It depends in large part on the fuel being burned. If the cruise ships are burning Heavy Oil then they're filthy. Same for GEBE. If they're burning more expensive Ultra Low Sulfur Diesel (ULSD, same as diesel cars/trucks in the US) then they're much cleaner. The efficiency of the generators (Tiers 1-4 in the US, Stages 1-5 in Europe) is also a factor, but any generators would likely be grandfathered to whatever the regs were on their install date.