There is a decent selection of quality B&B in Venice but one I highly recommend is the Venice & Venice :
Nestled in a tiny street (near Campo Santa Maria Nova) in the Cannaregio district (one of the most interesting of the whole island IMO), it offers first a great location : It is remote enough from the touristy stops to avoid noisy gatherings at night and “tourists-only” restaurants. Yet, it is walking distance from the Piazza San Marco (about 15 minutes walk), the Rialto bridge (less than 10 minutes walk) & the vaporetto stop which get you to the Burano Island or to the airport (less than 10 minute walk). Venice & Venice B&B is located on the first floor of a small building. It has only three bedrooms. Each bedroom has its own private bathroom. The largest bedroom is called the family room (pictured above). It is the brightest room and is worth the extra amount if you are on a romantic trip .
The rate normally includes the breakfast. (only the city tax is extra).
The rooms are comfortable (I actually tried two of the three rooms) and well-kept. No TV in the room ( it is NOT that type of place). You have free wifi (you even have two networks to make sure you always get a strong signal regardless of where you are in the place).
But the feature which, in my opinion, makes this B&B such a great lodging option is the welcoming of the owner. Monica is a true venetian woman and in addition to her great sense of hospitality, her knowledge of the city is going to make your stay a more pleasant one by your getting precious advice on which places to see, places where to eat,..
I was there for an entire week and can attest that I did not get a single bad recommendation from her. I recommend it without hesitation.