In my opinion in Italy it’s better to segregate diving from sightseeing . When you go diving you need to reach the diving spot, park the car/leave the train station, do the paperwork in the diving, wait for the scheduled departure time of the boat, set up the gear, do the briefing. Then you dive and after that you wash the gear, etc. etc. There are towns with very good dives that start close to the town center, for example Genoa, Naples, Trieste and also Venice if you are lucky with the visibility. But even there you need to dedicate at least a full morning or a full afternoon to diving if not the whole day