I haven't been to greece in over a decade but I was in italy briefly earlier this year. Generally I've found them to feel like pretty safe places to shoot, although I'd be a bit more wary of greece given the current climate.
I assume your cruise ship room will be fairly safe. With any luck it'll have an actual safe. Wherever possible I do try to find rooms with a safe so I can lock up my most expensive gear any time I want to go out without it. Failing that, having good insurance is helpful
Really the worst thing to be stolen would be your photos. On my 5d3 i've got twin card slots so I pull out the SD cards periodically and hide them in my dirty laundry while keeping masters on the CF. If you have a laptop you can figure out a similar backup strategy. Don't bet on cloud backup in either of those locations, hotel wifi is generally quite slow and cellular is pretty hit or miss.
I didn't have any major issue flying with a bunch of gear. The only airport that's ever given me trouble is Paris CDG where they made me actually take every piece of camera equipment out so it could be xrayed separately. That was a pain in the ass (especially to the people behind me)
My other advice would to be enjoy your vacation and not be so fixated on shooting that you miss what's (probably) more important.