Venice Marco Polo Airport to the San Cassiano Hotel, Venice, Italy

Alilaguna Water Bus to Rialto, then walk
or
ACTV regular bus (either line 5 or Air bus) to Piazzale Roma, then a vaporetto to San Stae stop on the Grand Canal (then walk).
or
Water Taxi boat (expensive) to the dock closest to your hotel.

see details:
http://www.veniceairport.it/en/transport/venice.html

Or follow simar instructions from your hotel's website after you click "Location" on their home page. http://www.sancassiano.it/location/?cookieChecked=true