Some of the morning trains from LA stop in Old Town, which would be a little closer to PB, saving you time and money if you don't have any checked luggage. The cab stand is on the east side of the tracks, towards the state park. I would think, but am not sure, that there will be enough activity in Old Town on Christmas day by the time you arrive to justify taxis waiting at that stand.
If you're a mass transit type of person, another option is to get off at Old Town and then take the #30 semi-express bus to PB. It would travel the same route as the taxi mentioned earlier. You'd get off just after it turns onto Mission Blvd. from Grand Ave. and then you'll have a four or five block walk to the Blue Sea Lodge. You can also take the slower #8 bus from Old Town that would drop you off just a block from your hotel. Both buses only run every 30 minutes on weekends. There will be a 2 for 1 promotion on Christmas day, so total fare for two people will be $2.25.
FYI the SWC frequently gets into LA early, sometimes as early as 7:15am, so you can simply hop on an earlier Surfliner train to SD and be dipping your feet in the ocean earlier than scheduled.
Let us know what you ultimately decide to do and how it turns out. Enjoy your vacation in our city.