>>> The average EU citizen living in "centuries old city centers" emits ~5.66 tCO2eq/year
As the GP said, that excludes the flight to get there. Also it does not include going to a swimming pool everyday, and probably not crazy 15°C/60°F AC [1]. It's not a good comparison of old style turism with crouse turism.
[1] At home I use the AC at 26°C/79°F. It's insane to use a wool pullover in summer just because someone likes the AC at 15°C/60°F.
Cargo ships don't need much power output in port. The main power plants are shut down, usually it's only the (tiny) auxiliary generator unit in use.
A cruise ship however? That one runs orders of magnitude more electrical stuff all the time, and that's a loooot of fuel consumed.