I was just in Venice, Rome and Florence, last month, and in 2004 was in Venice and Sorrento area in late June. I remember it being pretty hot on the Amalfi coast back in 2004. I didn't get to see all of it thanks to a problem with our rental car. We took the train from Salerno to Florence then. We dropped off the rental car, got to the station and it was 30 minutes until our local train took us to Naples for our high speed train to Florence. All we had to do was walk up one flight of stairs. We missed the train. Why? Because it was the only flight of stairs open and they were narrow. So the Italians getting off trains and coming down had to stop and argue with the Italians going up to get on a train. 45 minutes of gridlock. Then when we did get up to the platoform level it turned out all the trains were very late because it was a railroad strike. They are nice enough to announce this ahead of time, of course not in any place a typical tourist would look.
Still I would reccomend travelling by train. You could take a night train from Naples to Milan, then it's just an hour or two on to Venice, and that would be in the morning when you probably couldn't check into your hotel anyway. The eurostar trains are very nice and comfortable, much more so than Europe's budget airlines. Also check carefully with the budget airlines, most are 20kg of CHECKED bag weight and one carryone, some are even less than that. If you go the budget airline route take a look at www.whichbudget.com
to search through many of them.
While you're in Sorrento it's not that far at all to go to Pompeii and probably even better, Herculaneum. Pompeii was miserably hot back in June 2004 so get there as early as possible. I also went again last month and it was a rainy day. As soon as the sun broke through the clouds it was again hot and muggy. Same story at Herculaneum, a bunch of old building 60 feet or so below current street level with hot sun beating down on it all day. It gets toasty.
Venice I also remember as being hot in June. I know we had a hotel with AC and were very glad we did. The regular Doge palace tour was no trouble to get into but the Secret Iteneraries one was booked solid. Oh well didn't sound like much to me, and they won't let you photograph most places in the Doge palace anyway.
Besides, the best photography in Venice is off the beaten path. Get a decent map, then just start walking and try to get lost. Then start looking at the signs at the ends of alleyway and streets pointing to known landmarks, Piazza San Marco, Pier Rialto etc., along the way you'll see all kinds of people, shops, architecture, etc. to photo. As for stink from the canals, never, and they flooded when we were there last month at high tide. You might be walking and go around a corner and it smell awful but it's just like any old city, here and there the sewer is backing up on any given day. I'd worry more about stepping it dog doo as there's far too many people there that walk there dogs and don't pick up after them.