Thank you for all sharing all this information.  I didn't think it would involve several, not all simple possible solutions.  I've been looking at maps of the city and its layout.
Sorry I didn't make it clear.  I don't really need to save money, though I'll be in Venice (Dorsoduro) for 22 days.  I'm just really interested in the easiest way to get to Piazza Reale after a long plane trip Seattle - Frankfurt - Venice.
My landlady wrote to me via WhosApp or the plaform I rented through ways of getting to the apartment but I wasn't able to find the message.  I had read through it but was still not very clear.  I asked her again yesterday to send me directions but haven't heard from her yet.  She had mentioned a water taxi (from where exactly?).
Previous guests/tenants had mentioned that she had met them somewhere.  Addresses in Dorsoduro, apparently, do not have street numbers.  I don't recall if she suggested meeting at Campo Santa Margherita.
I'd like to have a clear notion of how to get there way before I leave Seattle.