Venice to Florence by train

If you're staying on the islands (you definitely want to be here) in Venice, that's correct. If your hotel is actually in Venice (Mestre), the mainland, you would begin from Venezia Mestre.

An easy way to check is to put your hotel addresses in rome2rio.com, and it will show you the station names, map & approx. time the route takes by various travel methods.

Charlie, yes, those are the correct stations. Venezia S. Lucia is the one on the island. Mestre is the one on the mainland which you DO NOT want, all trains from Venezia S. Lucia call at Mestre as their first stop.
And Firenze S. M. Novella is the main station in the centre of Florence.
There are frequent high speed trains between the two, times and prices on the Trenitalia site. The earlier you book, the better chance of getting a cheap ticket. But, when you book you must pick a train and the ticket is only valid on that train (it will say date, time, train No., coach No. and seat number on the ticket). You can get a "print at home" ticket on the website, nothing needs to be posted to you.

Charlie,

For trains between Venezia S.L. and Firenze SMN, you can also use Italo Treno high speed trains, which are a competing network to Trenitalia. Travel time is about the same, so use whichever one has the best prices and the best timing that fits your circumstances. As Chris mentioned, tickets for the high speed trains are specific to train, date and departure time so be sure of your choice before hitting the "buy" button.

You can purchase online tickets either from the rail network websites or from trainline.eu at the same price. I normally buy from the Trainline website and use the "print-at-home" option.

So the tickets this trip are best to book online before we go to Italy? How far out can you book them? I noticed it didn't show anything available in Sept 2017, but found times and prices for March.

Tickets for the fast trains go on sale 120 days in advance. The savings for committing to a specific departure can be significant.

JG, you can't buy tickets 8 months in advance. Checking three months ahead, there are super-economy fares for €29.

Thank you all for the replies and info. We are just trying to plan for the trip. Not looking to book any train tickets now, but wanted to know if this particular leg would be best to reserve online prior to getting there or just get tickets at the train station like we usually do.