Private Water taxi

Is your concern that you'll have a lot of luggage? The vaporetti are plenty reliable. (I assume that's what you mean by "the bus.") And there's no security or anything for the train, so you only need to be there about 15 minutes early at most, as long as you have your tickets already.

The ACTV run the public vaporetti in Venice. Fill in the departure and arrival stations, or click on them on the supplied map and you will see a 7:45 departure that will get you there in 36 minutes, half an hour before your train at 8:21. For me, that's a better deal than €55 for a short ride in a taxi. That's way sooner than I would need but it might make you more comfortable. The route 1 is a reliable and frequent line. At that time of the morning nearly all the passengers will be arriving in Venice and very few going towards Ferrovia. Go to the far right of the result and click the clock and you can see the run quotes and the following two. You will see that M-F there is a boat every 10 minutes, but the middle boat doesn't go on Saturday or Sunday. I don't think you said what day of the week you are leaving Venice. You may prefer the taxi down the Grand Canal, or even around the Guidecca Canal which has much higher speed limits, but I'd have breakfast and use the vaporetto.... up to you...

The concern is how long it would take us to get from the hotel to the vaparetto stop if we used that option, when it runs, how often etc. If we schedule a taxi, they come get us, we go.
walking slower than usual just now, and prefer ample time to find platform (and maybe purchase a snack for the train ride) once we get to station.

Regarding how far to the stop - I google mapped your hotel and it looks like its around the corner and down the alley to stop. Walking normally, maybe 2 minutes. Slowly, maybe 3 or 4? At Ferrovia you can't get a lot closer than the number 1 stop. Up the stairs into the station, or use the ramp on the right hand side. Either way, taxi included, you have to get up the slope from the water to the station. How high? Never measured. Maybe 10 or 12 feet higher?

You are worrying way too much. The vaporettos are frequent and run just fine. The Ferrovia stop is the train station. The vaporettos are easy to get on and off. Once at the train station, you are about 250' from touching the train engine. It's that close between the Grand Canal and the trains. Your vaporetto stop is the S Marco Vallaresso stop. Only the Number 1 and 2 vaporettos stop there during the day. It's a 36 minute journey by the #1 vaporetto which stops at every stop. The 7:35 vaporetto will get you to the train station by 8:11. Here's a LINK to the ACTV vaporetto schedule for the 7:35 vaporetto from the S Marco Vallaresso stop to the train station. A private water taxi to the airport normally costs 90+ Euros for the ride. With the train station only about 5 minutes away, the cost may be reduced. But, you can easily do this on the vaporetto.

Thank you both. Will consider these options carefully The slope will be fine.
Taking a taxi might allow us to have breakfast at hotel.

Catherine, it does appear to be a 10-minute walk from your hotel to the vaporetto stop. Note however, that you will have already arrived at your hotel and possibly have traveled around a bit. You have plenty of time to discover the walking path, distance and time between your hotel and the vaporetto dock. All of this being said, I hope that you have your train tickets in hand. If not, you are likely to spend time in line to get your tickets. If you don't have them, or you need them adjusted, don't wait until the last minute. You can take care of all of this before your departure day. Again, you are worrying way too much.

I'm thinking you really want to rent a water taxi. That's what it's sounding like. But Larry has an excellent point - You don't have to decide right now. See what your comfort level is once you're there. You are the master of your own domain. :-) Here's a video clip I took of the area in front of the train station from the steps, in case you're interested: Santa Lucia train station and the Grand Canal

THanks, all. We do appreciate the help.
No comments at all re: Consorzio Motoscafi.

I've used the consortizo Motoscafi twice with reservations. First time - no problems Second time - from Piazzale Roma - there is a taxi stand there, but it was confusing as to which one was ours. It took a few minutes to get this sorted out and I had to call the company. Likely also due to the fact that the taxi was prebooked. From Piazzale Roma it would have been easier to just get on the first taxi available. I would expect that for a hotel pickup, they would be on time. They were for our first trip.

I'd suggest waiting until you arrive to figure out your transport to the train station. As mentioned the vaporetto is fairly easy. If you decide to go with a private water taxi, ask your hotel to arrange for one. They will have folks that they routinely work with. The rates are in theory, set by the city and the motoscafi are very regulated. The rate is usually around 60 euro from most of Venice to the train station. By booking online you may save about 5 or 6 euro, not that much.

Thanks for this information.
The Consorzio Motoscafi website quoted 55 Euro; The hotel quoted 85 Euro...

OH.. and yes, we do have our train tickets already.