I took a look at the CGN timetable.
The map on that page makes the obvious suggestions visible. However, without knowing how much time you have, and what day of the week you are traveling, it is hard to make choices.
The route from Lausanne to Montreux ( or Vevey) would logically fit in your journey. Optionally, you could also consider routes N1 and N2, because Lausanne is the most obvious port along the route of your journey at which to transfer to a boat.
Those routes in the northern and eastern part of the lake would have particularly scenic nice views, but any ride on the lake has nice views.
Study of the timetable shows that that particular route (or sections of it) only runs on Sundays and public holidays. It is served by an old restored steam-powered paddlewheeler. However, if I interpret the timetable notes correctly, the steamer is not used on runs from January 8 to February 1.
Since the note is a bit obscure to me, I'd give them a call, or an e-mail. You would not want to find that the correct meaning is that NO boat runs on those days. ;-)
You can look at the map and the remaining schedules yourself. Any other route would require a deviation from your journey. Some of the routes available on weekdays would be a round trip to/from one location, such as the routes labelled N1 and N2.
The hours for those routes (N1, N2) clearly are designed for people commuting back and forth to work across the lake, and have a substantial gap in the middle of the day.
That's the best I can do. Possibly, someone else will jump in to add information. Otherwise, you'll have to work the boat timetable your self, since you know how much time you have, and what day. You can coordinate with the SBB timetable for your train schedule, or even use the advanced version to string together the entire trip, as noted in the link that I provided earlier in the thread.