We've just taken a round trip cruise on MSC Seaview from Civitavecchia followed by a week in Italy, with the itinerary Rome - Florence - Venice, flying back to the USA from Venice. It was just before the current heat wave. The cruise was from June 28 to July 5, and we flew back on July 13. It was hot throughout, highest temperatures in Florence (around 38C or 100F). We've managed to do a lot of sightseeing, but a lot of it was in air-conditioned museums or cool churches. Even though we liked our trip, we vowed not to repeate a Southern Europe trip in summer (since our daughter has just finished her 3rd grade, this could be a long time). Venice was cooler and generally better due to breeze along the Grand Canal, but of course more humid. I would not count on better conditions next June, or any June after the next one.
