If you plan to be in Pompeii, you would have one of the best sites in Italy (possibly the Mediterranean) close by with the Banco di Santa Croce (best reached from Castellamare di Stabia; Bikini Diving would be a good place, at least when I went a few years back). Most of the best diving in Italy tends to be on the islands, though. Your itinerary won‘t get you near Northern Sardinia or the great sites around Sicily (Ustica (Mr. Jump), Marettimo (BluTek are great) with lots of archeology dating back to the Punic War and some of the best diving I have seen anywhere, including tunas, lots of barracudas, turtles, the odd shark (longimanus and grey, if you don‘t mind hitting 100-130ft), Pantelleria (similar) etc.). If you are into big/pelagic fish, you may be disappointed in mainland sites. That said, I love diving the Med, it‘s just different from the tropics, but not in a bad way, and somehow Italy gets an undeserved bad rep on SB, mostly from people who haven‘t been there. I hear great things about the Strait of Messina (Scilla would be doable on your route) from friends but haven‘t been there. Keep in mind that the water may still be chilly in late May (I go dry in that season, having fought with a torn rented 5mm once and not wanting to repeat that - doable but cold), and in some places there could be a bit of current and/or swell. Do check it out when you are there, is all I can say. Have fun, above and below sea level!