Walk about 5 minutes from the hotel to the Tronchetto vaporetto stop.
Take ACTV Vaporetto Line 2 toward San Marco.
Get off at San Marco–San Zaccaria (or San Marco, depending on the service).
It’s then just a 2–5 minute walk to the Piazza San Marco. The boat ride itself takes about 25–30 minutes, and the total journey is around 35–45 minutes.