In the city of Venice, there are only a few water taxis with a lifting platform. They must be booked in advance (Venezia Turismo +39041240 27 71 or Consorzio Venezia Motoscafi: +39 335 230 896). But the Vaporetti and Sightseeing boats are accessible (barrier-free vessels are all signed). The Giardini and Arsenale Biennale can be reached that way easily.
Some bridges (routes are clearly displayed here) are nowadays designed with wheelchair access. So they are not an insurmountable obstacle anymore, because special lifts or ramps are installed.