Indeed, Italy is not among leading nations in the area of disability access--and Venice, of course, presents particular challenges for people who use wheelchairs (or if you have luggage and there are more than a few bridges between where you disembark the vaporetto and your hotel).
Unfortunately, after a guided tour of the Venice Biennale (decidedly inaccessible to visitors who are blind), I expressed the concern at a roundtable and the general inaccessibility of Venice was cited. I immediately responded that the arts could lead--could show Venice the way. (I'm in the process of preparing a formal access proposal to the Biennale.)
Nonetheless, Venice is a unique city that I had only experienced once before, many years ago. Come cruise its canals with me!