Transfer from Venezia Santa Lucia Train Station

What does your cruise line recommend? It is a major commercial shipping port that fought the inclusion of cruise ships for a time, so I don't thinknit is an easy DIY trek.
Alternatively, join Cruise Critic (free sister to atripAdvisor for all things cruising). They have 'boards' instead of forums and these will include cruise ports (which may have something about getting to Chioggia), or use the cruise line boards and ask.

There is a route via vaporetto to Lido, bus then ferry from here. This article describes the route but I could not confirm that it is current.

https://europeforvisitors.com/venice/articles/venice_to_chioggia.htm

I was reluctant to recommend the water route as Google maps shows it to be over 3 hours whereas by road should be under an hour. And then there would be luggage to contend with on the bus, boat,walk option.
Rome2rio website may have some alternatives.

Bus from Piazzale Roma which is a short walk from the RR station. They are scheduled every 30 minutes and take just over an hour.

The article I posted mentions the land route by bus from Piazzale Roma, but says it is “less scenic”. But it is only an hour 15 minutes.

Here is the schedule:

https://arriva.it/app/uploads/sites/2/2023/06/Linea-80-Orari-Lunedi-Venerdi-dal-12-06-2023.pdf

If they are starting from the train station, they will have to either take the vaporetto from Santa Lucia Ferrovia to Piazzale Roma, or walk with their luggage and take the steep Ponte Costituzione across the Grand Canal.

https://bridges.myengineeringsystems.co.uk/Pages/NotableBridges/VeniceCalatrava.html

Both involve a bit of a struggle if they have lots of luggage, so may not be too much easier than the vaporetto plus bus plus ferry option. But definitely faster.