Venice has a system of waterbus (Vaporetto) routes that get you around all the islands and they are very wheelchair friendly. The biggest boats, like the one in the photo to the left, run on routes  and  and display the blue wheelchair symbol. There are smaller boats on other routes which I did not see the symbol on but I managed to use every one that I tried and the crews on them were very helpful.
To access the waterbuses there is usually a small step down, as can be seen in the photo to the right, I had no problem getting down this but needed assistance to get up it. The boat men and ladies were quite happy to assist in this way. On the larger boats on lines 1 and 2 there is an open deck in the middle where you access the boat, if it is not busy you can sit here and get a great view but if the boat is busy you will be asked to go to the rear passenger cabin where there are a couple of wheelchair spaces with seatbelts if required. On the smaller boats, without the blue badge symbol, I was directed to sit next to the wheelhouse.
There are either ticket offices or ticket machines at most of the piers to buy your tickets, then before boarding the boat you hold your ticket near the validators. The cost of a trip on the boats is €7.50 and you can get on and off for up to 75 minutes from first validating your ticket provided you continue in the same direction, if you change direction a new ticket is required. However, for wheelchair users if you go to a Tourist Office (there is one near the bus station at Piazzale Roma) you can buy your tickets at a cost of only €1.50 and a companion can travel with you at no further cost.
The waterbuses within Venice, along with land buses, are run by ACTV, there are also waterbuses that run to and from Venice Marco Polo airport which are run by a different company called Alilaguna and which require different tickets. The cost of these depend on where you are travelling from, as an example I caught one at San Marco which cost me €15 for a single ticket. This includes transporting one suitcase. For these boats the crew will get a ramp to assist you on and off.
If like me you thought Venice would not be a wheelchair friendly city to visit you may be surprised. During my two days here I found it very easy to get around using the waterbuses and also the regular buses on land. You can get to many places by taking short hops rather than trying to cross the many, many bridges. Venice is somewhere I will definitely be returning to in the future.