first of all here is a link to the port map of Venice.
The Piazzale Roma is the last big place, where buses and taxis and so on can drive. From there you can get the shuttle to your ship or go by taxi. From the P. Roma you can go by a public waterbus (vaporetto) Linea 1 for one stop to the main station (S. Lucia), marked on the top of the map. From there you have some 200 meters to the hotel "Mercante di Venecia".
I stayed there for some times, friends too. It is a small but "great" hotel hotel. I am looking for a link. Until later