I do dive. And I've been to Italy. But I've never gone diving there. Here's my input...
First, this is not the right place to get a reasonable answer to your question. Sure, there are a few divers here on this forum, but diving is not its focus (not at all). If you have a question about visiting Italy - eating, sleeping, sightseeing, trains, shopping, etc. - then this is the place. But for diving in Italy, get over to scubaboard and you'll get useful info from actual divers.
Second, the answer to the question "how's the diving in (insert any location in the world)?" depends mostly on you - yes, you. Most divers would tell you that any day diving beats a day spent working or sitting at home, etc. But that's a useless comparison, because - assuming you have a limited number of days in Italy - you are not trading a day at work for a day diving in, say, Livorno (just to pick a place at random) - you're trading a day in Tuscany for a day underwater near Tuscany, and that may not compare well to other ways you could spend that day in Tuscany.
Most of all, it would depend on what kind of diving you are used to. If all you've ever done is dive in a rock quarry or a lake in Minnesota (cold water, limited visibility, not much marine life), then yeah, I bet anytime you can get in the Mediterranean you would feel like you were in heaven. OTOH, if you're more used to diving tropical Indonesia or remote Pacific atolls (the most biodiverse and spectacular marine environments in the world), then you're probably not going to be terribly impressed by what you see beneath any Mediterranean waters.
Personally, as much as I love diving, going scuba diving is not high on my list of things to do in Europe - because I'm spoiled. After diving Raja Ampat, Palau, and other remote places (and even some less remote places like Cozumel, Belize and Baja), I think I'd rather spend my days in Europe doing things that are unique to Europe and aren't "better" somewhere else. This is, admittedly, in some ways a bad problem to have (and in other ways not). FWIW, I've been to more than a few places around the Mediterranean (including Sardinia) and I never felt like hauling my scuba gear al the way there would be worth the trouble (I did take my mask and snorkel to Sardinia and enjoyed having them, but never felt like I would trade a day on land there for a day diving - although I know others would disagree). But I absolutely would (and did) drag all that stuff to Bali or Koh Lipe.
But I think in general, you will find the waters around Italy - while sometimes warm, clear and pretty - not very full of marine life compared to what you can see elsewhere (even as close as the Red Sea). If that's OK for you - you don't care if all you see are maybe a few tiny fish here and there and you just want to blow bubbles and enjoy being underwater - then it might be a good way to spend that day.
Me, I'd probably rather spend that day in Tuscany exploring a hill town or sampling local wines, and I'd save my diving for the next trip to the South Pacific. But it all depends on what you're used to. No disrespect intended towards the undersea attractions off the Italian coast.
Bottom line: get over to scubaboard for input from divers. For anything else about Italy, this is the place.
Good luck.
