Train from Milan to Venice & Vaporetto to Hotel Serenissima

Your hotel would be the best source for directions.
The vaporetto stop is at the train station.
Depending on how much you'll be using the vaporetto , you may want to purchase a multi-day pass, and yes, you can buy it at the train station. I don't know of any advantage to pre- purchase, as you still have to physically pick up the pass, the last I checked.
Have a great trip!

Yes, there is a vaporetto stop outside the train station. You can take either line 1 or 2 to the Rialto Bridge stop. There's no need to buy tickets in advance. The train station and the vaporetto stop will have kiosks to buy tickets. Depending how long you'll be in Venice, it might be worth getting a vaporetto pass since a single ticket is 9.5 Euros.

Most hotels in Venice have arrival directions on their website
Scroll way down
https://www.hotelserenissima.it/location/

Be sure to validate your vaporetto ticket each and every time you board

Here is detailed information about the Venice train station - including how to buy train tickets.

https://www.seat61.com/stations/venice-santa-lucia.htm

These are directions on how to get to your hotel copied directly from your hotel website:

From Venice Santa Lucia station Take vaporetto No. 1 or 2, stopping at Rialto (not Rialto Market)

From the Rialto stop Line 2 dock: once you disembark from the vaporetto, turn right and cross the bridge, Ponte Manin. You’ll find yourselves at the line 1 dock. Turn left onto Calle Bembo (it’s right in front of you if you get off at the line 1 stop). Continue beyond the intersection, where Calle Bembo becomes Calle dei Fabbri. Turn right onto Calle del Magazen, then left onto Calle Goldoni where you will find Hotel Serenissima.

You can go to Booking.com and look up your hotel. Click on the map and magnify it to show the streets/alleys that will get you to your accommodations.

Hotel Serenissima is not on a canal, but it appears the Rialto vaparetto stop would be the one to use. It's a good location and a relatively short walk to St. Marks Square.

When we were there in June, my 12 year old granddaughter asked me where we were. I said, "Honey, I don't know where we are and I don't know where we're going but we'll eventually get there." Venice is kind of like that. I've been there 10 times+ in the last 54 years. We walk the city and love the back alleys and things we run up on.

The Ferrovia vaporetto stop is directly outside the train station, but it has 5 different docks—-A to E from left to right as you see them from the train station steps. You can see them on the map here (you can enlarge it for a better view):

https://www.venice.nu/maps/vaporetto-stops/

Each dock serves a specific vaporetto line and direction. You lowland Line 1 or 2 going towards Rialto. There will be a board showing which dock you want, and it will also be on the schedule and the dock itself. According to this schedule, for Line 1 going to Rialto you want Dock E:

https://www.visit-venice-italy.com/water-bus-venice-line-1.html

Line 2 uses Dock B:

https://www.visit-venice-italy.com/water-bus-venice-line-2.html

But confirm this on site in case those schedules are updated.

You must have a ticket and validate it before you board. The ticket will open the turnstiles to let you into the waiting area on the dock. You can buy tickets from the kiosk to your left as you descent the steps, but there may be a long line there. It is easier to buy your tickets from the machines nearby.

I will repeat, make sure you validate each ticket with the little machine—-watch others doing this so you know what to do. This blog has a good photo of the validation machine:

https://robynmulgrew.wixsite.com/a-blog-about-venice/single-post/2016/03/10/validation-and-vaporetti

Thanks to you all. Extremely helpful information!