Departing from como, which destinations around the lake can be reached via fast ferry?
All the main mid lake villages can be reached on the fast ferry. See link to current timetable which shows the destinations in more detail. The fast ferry is not the best way to enjoy the lake however. The ferry is enclosed, and although the windows are quite large you are going too fast to enjoy the view, and you may not get a seat near a window. It is primarily a commuter service.
It is much nicer to take one of the slow ferries and enjoy the journey.
The link shows the route heading north, the fast ferries are in red, all the timetables can be found here
The current timetable refers to the period up to 26 May, the summer timetable should appear in a few days.
There is also a bus running up the west coast of the lake, which you could take to or from the midlake area, then use the more frequent ferry service in the mid lake area. The bus timetable can be found here:-