you'll love it. They use the high speed Eurostar Italia rolling stock--running time between Venice and Florence is roughly 2 hours 45 minutes (the high speed 150 mph line in Italy is between Bologna and Rome, Florence is roughly 55 miles south of Bologna).

Trains go at least one an hour--Santa Lucia station in Venice is adjacent to the Grand Canal, and the public transit "vaporetti" call there frequently (no more than a 10-15 minute ride down to the Piazza San Marco). Santa Maria Novella station is almost in the heart of Florence, probably no more than 1 mile from the Ponte Vecchio. Enjoy.