Welcome to My Swiss Alps.
Please look at this map upon which I have searched for Titlis:
map.search.ch/?q=titlis&near=662000%2 C190000&pos=672896,195 072&z=4
If you zoom in a lot, you can see the rail lines.
The train to Titlis from Luzern goes from Luzern to Hergiswil, then takes a branch line via Stanstad and Stans along the valley toward Engelberg via Dallenwil . Engelberg is the base town for Titlis.
Durng some parts of the year, it is possible to take a boat to Stanstad, and pick up the train there, although people usually take a train all the way. Between Luzern and Stanstad you have nice views of the area around part of the lake, and some lake views. At any time of year, the boat service is less frequent than the trains; in the winter, boat service is significantly more limited everywhere on the lake. Basically, the only route running in December is the one on the main part of the lake to and from Flüelen.
To understand the schedules, use the timetable
and take advantage of the "advanced search" feature to limit the choices to boats. See attached screen capture to see summer service on a Tuesday. The search for Luzern to Stanstad, limited to boats only in "advanced search," returns boats to Stanstad See, which translates as "Stanstad lake."
The same search for December 6 shows no boat service.
I have also shown service (attached) from Stanstad See to Engelberg in the Summer. Note a ten minute walk from the lake boat landing to the train station in Stanstad and a change of trains in Stans.
A check of the lake boat Winter schedules (scroll down) shows no service to Stanstad at that time of year:
www.lakelucerne.ch/fil eadmin/lakelucerne/con tent/Dokumente_Downloa ds_PDF/01_Fahrplan-Preise/01_Fahrplan/Sai sonaler_Fahrplan/2016/ Fahrplan_Winter_Okt_De z_2016.pdf
Checking the Autumn schedules (scroll down) shows service to Stanstad:
www.lakelucerne.ch/fil eadmin/lakelucerne/con tent/Dokumente_Downloa ds_PDF/01_Fahrplan-Preise/01_Fahrplan/Sai sonaler_Fahrplan/2017/ 2017_Fahrplan_Herbst.p df
As far as I can tell, you cannot include a boat trip in any kind of straightforward journey from Luzern to Titlis on December 6th, or other days in December.
Perhaps you meant Pilatus?
Boat service to Alpnachstad, at the base station of the cograil train to Pilatus Kulm, runs only until November 20, 2016, resuming in the spring or summer. (see the Winter lake boat schedule, on the right.) Also, cograil service on Pilatus is suspended during a good part of the winter.
The cable car to Pilatus Kulm, via Kriens, which does run in the winter, does not have a route along or on the lake as part of the journey.
Although I mistakenly searched for journey TO Titlis including a lake boat ride, the same conclusions are true for the return journey. If you wish, you can try your hand at the timetables using my examples as models to confirm that.
As far as i can tell, you cannot make the journey you requested, even if you meant Pilatus. If you want to check at the tourist information office by track #1 in Luzern main station, you can try to prove me wrong. ;-)
It is possible to go by boat from Luzern to Vitznau and then cograil train on the Rigi to Rigi Kulm in the winter.