Cheapest accommodations in Italy

Check booking.com (filter by price) and read reviews before you book. I haven't heard of shared hotel rooms other than dorm rooms in hostels. Some cheaper hotels have shared baths.

I don't use airb&b, so can't advise about that, but I do know that some are cross-listed on other booking sites, so the prices would probably be similar.

Are there even cheaper accommodation options available aside from
hotels and Airbnb? I am aware of hostels, but I'm under the impression
that most of them are shared rooms.

A fair number of hostels have private rooms as well as shared dormitory rooms. They do cost more but are worth taking a look at if that's your preference.

As far as low-priced hotels, be SURE and check reviews AND actual locations? There can be reasons why they're so inexpensive, such as they're way out on the edges of the city or they're not very nice places to stay. That isn't always the case, particularly during low season, but I've seen tourists who thought they'd gotten themselves a great deal only to find their accommodation a loooong way from city center in an unattractive area with infrequent or expensive public transit. Check addresses on a map!

What Kathy said. Booking sites don't vouch for safety or comfort, but if there are lots of complaints, they may refuse to list the place.

I know a nice B&B in Napoli a few blocks from the ststion, post if you want info.

dz,

Sharing a dorm room in Hostels is not really that bad, although it does take some "adjustment". Those will usually be the cheapest accommodations anywhere. The Hostel I normally use in Rome charges €35 per night for a four-bed dorm, and there's an ensuite in each room.

Ken,

Hmm I see. I may consider that then. But how do you know you can trust those you're staying with? I need to leave my suitcase in the hostel during the day. Are there usually safes you can store a large suitcase in?
I'm more concerned about people snoring and making sounds when they sleep. But I can just bring earplugs for that.

Have a look at the EuroCheapo listings for Florence: http://www.eurocheapo.com/florence/ and Venice: http://www.eurocheapo.com/venice/

This website also has some good tips on neighborhoods, so even if you get an Airbnb, it will help you with locations.

Many hostels provide lockers but you often have to provide the lock. Theft is somewhat common in hostels, so I have slept with my money belt and taken it to the shower.

You're probably finding some off-season rates now, especially for the coast. In my limited experience of looking for single rooms on Airbnb, I never chose one because they either weren't cheaper or not enough cheaper for the trade-offs in location and amenities. Close to a train station is not always the nicest neighborhood but not necessarily unsafe and definitely handy.