Ca' delle Acque is a small yet charming Bed and Breakfast in Venice center, just between S. Mark's square and Rialto Bridge.
It's very private, just one room per floor, and quite cheap for an accommodation in the historical center of Venice. The service is helpful and friendly and our room was clean, with private bathroom. No special features like WIFI or internet connection, only LCD Sat TV, but a good place to rest a while between all the sightseeing.