Twain remarks that the gondola he rode in belonged to the Grand Hotel d' Europe, so I assume that is where he stayed while in Venice. I did not find a listing for this hotel on Google but I did find the Hotel Londra Palace of Venice operating in 1853.

The Hôtel Louvre et Paix (a.k.a. Hôtel de la Marine) is a historic building in Marseille, France. Dedicated in 1863 as a luxury hotel, it was used by the Kriegsmarine during World War II. It now houses city administration offices and a C&A store.