I'd highly recommend the Virtu Ferries Sicily-Malta fast catamaran service (the second largest catamaran in the world).
It's not the cheapest in terms of ticket price, but the onboard experience is fantastic with an excellent selection of reasonably priced food and drink. There was also a generous amount of comfortable seating, with a viewing lounge forward and observation deck aft. The ship never felt crowded so it was a very pleasant crossing. You can upgrade to a premium class, which gets you refreshments and access to your own deck. You also get the dramatic/historic entrance to Valetta harbour, plus presumably good views of Mount Etna if you travel to/from Catania (I did Pozzallo). Also, the Med is a great place to sail for a few hours in summer.
The Sicily-Calabria train ferry is also one worth doing (ideally on the train), it's well-documented on this forum already. The other non-train ferries on this crossing, I have done a lot, but they aren't as exciting and are a real mixed bag of ships/catamarans.
On Lake Maggiore, Italy the ferry from Italy to Locarno is a nice ride with bonus novelty for being international. It's a beautiful place, with pleasant towns too. I also liked the fast hydrofoil services for the novelty. I think these operate on Lake Garda too, but the scenery there is less dramatic.
The San Francisco bay area hydrofoils are very impressive and give great views of the cityscape and tourist sights such as Alcatraz Island, the various bridges and wildlife such as sealions, seals and dolphins. They are also great for accessing hotter and sunnier areas of California.
In the UK, I've only really done the Dover-Calais ferry which is okay, plus the Dartmouth/Lake District steamers and the rope car ferry thing, which are all pleasant enough.
I've done a lot more cruises, including one where I did a tour which took us on the bridge. Most impressive of all was a submarine trip in Barbados, but I realise I'm well beyond the scope of the thread now, so I'll just say it was stunning! In terms of great places that I’ve cruised; Santorini (can be accessed by ferry), Kefalonia and St Kitts & Nevis were stunning. I very much enjoy the tranquility and calmness of being at sea.
