I'd like to go Scuba Diving in Northern Italy and would prefer a place with good visibility and great marine life. I'm an Open Water Diver.
The west coast (the one on the Mediterranean Sea) is definitely a better choice compared to the east cost for scuba diving.
If you are planning a trip to the north, the best area for diving is the natural marine park of Portofino.
The dives in the marine area are gorgeous. Not only they are full of life such as groupers, dentix, barracudas, ... But in several places you can also spot some red corals.
The area is a little bit pricey, especially in high season, however the dives are absolutely worth. You can find an accommodation in Portofino, Santa Margherita Ligure and Rapallo that are exactly on the front of the marine area, or if you prefer something cheaper you can check other cities 20 to 40 minutes away such as Bogliasco, Genova.
The entire coast on the Mediterranean Sea offers great dives, hence you can dive in several other places in Liguria or Tuscany (the North part of Italy), however the marine reserve of Portofino is something outstanding the standard quality.
Some dives may require an advanced certification because of the depth, however there are many dive sites accessible to an OWD.