I did a similar cruise last year, visiting all those cities, except Venice, plus several more. I took my Canon 40D, 70-200 f4L and 10-22 f4 lenses and some extension tubes. The long lens was rare used and not often carried, as it seemed to not be required. I did use it once or twice for close-ups with the extension tubes, but I also used the wide lens, which will focus down to a couple of inches.
As a side note, a couple of years before, I found myself hanging over the edge of the ship, with the camera in my hands, and realized that that was a very dangerous position for the camera. I then added a wrist strap rather than the next strap. It seems to be much easier to carry the camera on the wrist. The strap allows me to just barely hold onto the camera without possibility of losing it, either by dropping or by pickpocket, although I never felt there was any danger of the latter.