Several years ago, we did an Adriatic cruise on Seabourn which started in Venice on October 31 and went to Slovenia, Croatia and Greece. We had absolutely glorious weather. Yes, Venice was rather under water, but it was into see how the place coped with the wooden platforms and it was actually kind of fun.
So, perhaps we were just lucky with the weather, but it was wonderful to see places that were not mobbed with tourist crowds.