Rome to Venice high speed rail

Not difficult at all

Where is your hotel?
You may be able to walk or take a vaporetto rather than expensive water taxi

If you search this forum you will find instructions for all methods of transportation in Venice

Here are some threads for you to read
https://search.ricksteves.com/?button=&date_range=6m&filter=Travel+Forum&query=Venice+transportation+&utf8=%E2%9C%93

All train travel in Italy is fairly easy so I am not sure what you are asking. Italiarail has a direct about every hour between Rome and Venice taking about four hours. You did not indicate your time schedule. (For some reason I could not access trenitaila - the national rail company of Italy.)

Without checking schedules I am not sure that there is a direct train to Venice from Rome. I will look later.

Both Trenitalia and Italo Treno run hourly trains between Venice and Rome. taking 4 hours for the journey

Our hotel serenissima is across the canal from the Santa Lucia train station. Does anyone know how to get to that hotel from the train station

There are direct Frecciarossa trains many times a day. Very nice trip of 4 hours. Buy your tickets in advance for a substantial discount , although discounted tickets are non-refundable, much like airfare. Super easy. Be at the station about 20 minutes before departure and watch the board for the track number to be displayed, then go immediately to the train to board.

Depending on where your hotel is, as mentioned above you can walk, or you can probably take a vaporetto (water bus) for far less cost than a taxi.

Our hotel serenissima is across the canal from the Santa Lucia train station. Does anyone know how to get to that hotel from the train station

Walk across the bridge. You can easily look at routes on Google maps. Google maps will be invaluable to you in Italy.

Have you looked at your hotel’s website?
Most have directions for “how to arrive”

Is this your hotel?
https://www.hotelserenissima.it/

If so it is not located where you have said

Bottom of this page has arrival instructions
https://www.hotelserenissima.it/location/

Our hotel serenissima is across the canal from the Santa Lucia train station.

Are you sure? The only Hotel Serenissima in Venice I can find is located nowhere near the train station. The hotel's website describes it's location: "The strong point of Hotel Serenissima is undoubtedly the strategic position in which it is located: in the heart of Venice, less than a 5 minute walk from Piazza San Marco and the Rialto Bridge. You can easily explore the historic center on foot and then, from the nearby Rialto and San Marco stops, get on a vaporetto (waterbus) to reach the most distant areas of the city and the other islands of the lagoon."
https://www.hotelserenissima.it/location/

You would need to take a vaporetto (water bus) from the train station, walk the entire distance, or take a water taxi to your hotel.

https://www.google.com/maps/place/Hotel+Serenissima/@45.436975,12.3232758,15z/data=!4m9!3m8!1s0x477eb1da0884821f:0x3ea431dd7b706f2b!5m2!4m1!1i2!8m2!3d45.435379!4d12.3357105!16s%2Fg%2F1th4874n?entry=ttu&g_ep=EgoyMDI0MTIxMS4wIKXMDSoASAFQAw%3D%3D

Take the vaporetto to Rialto. Watertaxis cannot get to the hotel any closer than the vaporetto.