Nigel
I an not sure I would consider Venice a particularly Disabled Friedly place as some of the bridges are quite steep and you often find steps. Despite that I reckon you could still see quite a bit of that wonderful city. I don't think you would not have any problems on the big ferries that go from Punta Sabbioni to St Marks and out to the various islands as access from the landing points is quite good. Some of the smaller boats could be a problem. I would suggest, if you can, to avoid the German school holidays becaue it is so busy as Venice is a favourite destination as its as near if not nearer than the South of France. Have you don't a search of websites regarding disabled access to Venice?
David