Venice cruise port to St. Lucia Station or Mestre

A water taxi is not a vaporetto. The first is like a land taxi which will cost about 50 EUR just to set foot in, the latter is like a city bus, but more expensive, like 7.50 EUR per ride.

I guess it depends on your physical condition and how much luggage you have. Take the People Mover to Piazzale Roma (about 2 EUR I think) then take a look at the bridge. From there you could: 1) Walk over the bridge to Santa Lucia station. 2) buy a 7.50 EUR ticket and take the No 1 vaporetto one stop over to the Santa Lucia. 3) Take a land taxi to Mestre station, costing about 15 to 20 EUR.

Edit - You can make that decision when you get there. Here is a photo of the bridge from your perspective. Note small steps. Look to the left and you'll see the vaporetto dock alongside. Rotate to the right and you'll see the land taxis lined up.
https://www.google.com/maps/@45.4384665,12.3190858,3a,75y,24.48h,71.84t/data=!3m6!1e1!3m4!1sBvPIRpNmcpmsZVt6ExbRKA!2e0!7i13312!8i6656

We took the People Mover from the port to the Piazzale Roma where you can take a bus to the airport. There is a bridge (there was in 2010 when we were there) with stairs that goes over the canal to the St. Lucia Station. We were in our early 60s then and we carried our luggage over that bridge. Since the bridge had steps, rolling the luggage was not an option.

I didn't think it was strenuous then and could do it again today, now that I am 71, but to some it could be strenuous.

You can take the land taxi too Mestre. We stayed one night in Mestre after going to Florence on the train, before flying out of the Venice Airport. I was not pleased with that experience, but you plan to go to Florence, so that is one way to do it. You will save on a taxi if you walk over the bridge, depending on your fitness to carry your luggage.

I have read different opinions on the easiest way to get from the
cruise port in Venice IT to a train to Florence. The walk across a new
bridge to St. Lucia has been described as strenuous for
“seasoned”travelers.

What do you mean by "Seasoned travelers"? Old or well-travelled? We did not find it to be strenuous, but it was slow going with wheeled luggage.
Here is a picture of the bridge between the St Lucia train station and P. Roma.

https://www.flickr.com/photos/165352950@N04/shares/5443R1

It was wide steps. So we could roll our bags several feet, pull it up one step, roll 5 feet more, pull bag up one step, etc.

Thank you for the detail- helps greatly. By “seasoned” I meant retired, experienced travelers who know enough to ask questions so we are prepared.

Unless you have significant mobility issues and/or are packing very heavily I would not call this strenuous. That said, taxi to Mestre might be a way to push the easy button.

Think of a vaporetto as more like a bus than a taxi.

The bridge is controversial. As I crossed it a couple years ago, a man in his 60s was walking as well. The steps are only about 3 or 4 inches high and are about 18 to 24 inches deep. It is artsy, avant garde, and potentially dangerous. The man missed a step and threw out his back. It was obvious that he was in a lot of pain. It's not overly strenuous, but rather it is non-standard stepping, and therefore you need to pay attention.