I live very near to Venice, and I am sure that it is forbidden to get into the canals, however more to the point you REALLY wouldn't want to! Cold and filthy water that is full of all sorts of nasties!! Yuk!!! You can however dive off jesolo and further down the lido is caorle and eraclea. To be very honest with you, there isn't a lot to shout about, I have only done a couple of dives off jesolo but each time there has been very bad vis and very little sealife, saying that I know people who go frequently and come back happy..! I haven't done much diving in the rest of italy as we tend to go further away, but I did do a weekend liveaboard in Capraia and that was wonderful, the boat wasn't great the dives were lovely. Great vis, comfortable temp and lots to see- So I would have to say by all means try the diving in Italy but go further south!!