Some guidance please on where to stay in Venice for walking restricted person

For most access to transport (should you need to get to Venice airport) something near the train terminal is most convenient.
Also, may we briefly discuss transport terminology? A water taxi is a private hire, whereas public transport by water is a vaporetto. Both are options, but on the ground,it helps to be clear if asking a local.

Hotel Ala would be a good choice for lodging. It is very short flat walk from the Giglio vaporetto stop with no bridges or stairs literally 2 min
and less than 10 min walk to San Marco also with no bridges or stairs ( as I recall)

Although you don't mention a wheelchair, this website might be useful : https://www.veneziaunica.it/en/content/accessible-venice
Look at the "without barriers " highlighted in red.

We enjoyed staying at Hotel Ala. However, there are two bridges between it and San Marco Piazza. We stayed there April, 2024, so I think my "bridge memory" is accurate. The bridges are manageable with bad knees, but you might want to have a hiking pole (not too hard to buy in Venice - poles aren't allowed in carry-on luggage). Tourists often crowd the handrails and many are oblivious about needing to move away so the "bad knees" person can access the rail. Most of the bridges seemed to be about six steps up and these pedestrian bridges are everywhere in Venice.

Edit to add: You don't say what the walking issue is, but if you are thinking about doing a knee or hip replacement, do that BEFORE you visit Venice. I had my knee done about ten months before our Venice visit and had no problem walking around Venice. If I had tried to do Venice before the knee replacement, I would have been painfully frustrated by all of those steps up and down the many little bridges over the canals. The two main bridges over the Grand Canal involve at least twenty steps up, then down. I probably would have spent most of the visit sitting in a campo while DH enjoyed exploring. Everywhere you will have to navigate around tourists blocking the hand rails.

Thanks Cynthia
Our last visit was 2019 so my memory is definitely a bit hazy!

Playing on the some of the other posts, is that hotels don't necessarily have elevators to the ground floor. There may be a flight of starts to the elevator lobby, or from the street to the lobby, so considering the ones noted in the Accessibly Venice links would be prudent.