How or where do I get a public water taxi from San Marco to train station in Venice please?

I think you mean a vaporetto?
Water taxis are private and expensive ( and not necessary for this route)

Just walk down to the water and you’ll see the vaporetto docks. Buy your ticket at the kiosk
Don’t forget to validate before boarding.

Give yourself plenty of time, vaporettos are quite slow.

You mean a Water Bus, aka a Vaporetto? And Santa Lucia Station?

This is the interactive map of water bus lines in Venice:
https://actv.avmspa.it/sites/default/files/avm/navigazione/MAP/interattiva.html

This is the PDF version to download:
https://actv.avmspa.it/sites/default/files/mappa_linee_di_navigazione%20_luglio_2022_web.pdf

The train station is the blu train icon near the Ferrovia (railway) docks, water bus lines from #1 to #5 stop there

Thank you both for your help! Take care!
SH

Hey Susie-just realized this is you!
Hope things are going well

You can read more here: https://europeforvisitors.com/venice/articles/venice_transportation.htm