I love Venice - went there for my husband’s 40th 8 years ago (eekkkkkk!!!)
There are various ways to get from the airport to the city. Most (if not all direct ones) go on the water.
There are a number of water buses that I would imagine are accesible for people using a chair. Since going into a chair for some trips I have taken similar water buses elsewhere and they were fine. But I don’t know if they would be suitable for a power-chair user who had difficulty in transferring.
I would be surprised if the water taxis would be accesible. They are very cool but incredibly expensive - look like speedboats from the 1950s
Lots of hotels have their own water transport which is the ultimate film star way of arriving (did you see how George Clooney and his wife went into Venice before their wedding?)
At the other extreme, there is a bus but this is a long journey and you don’t get the fantastic sight of entering Venice across the lagoon
Get a decent guide to Venice and it will give you the low down (Lonely Planet usually has a good section for travellers with disabilities).
To be honest though, Venice would not be a place that I would choose to go to if I was using a chair. Travelling around the city is very dependent on using the Vapereto up and down the canals and I found them quite difficult to get on and off even pre-DX. The streets are cobbled and have lots of little steep bridges over the smaller canals.
Most importantly, at a lot of times of year, a high tide causes an Aqua Alta where much of the city is flooded and you can only get across St Marks Square by walking over duckboards which are quite high off the ground. (The Police on duty during Aqual Alta do wear the most sexy thigh-high patent leather boots so they can walk across the streets and the square So there are some compensations )