We had a very nice morning tour of LA and Hollywood with a private shuttle service, and arrived at the ship in plenty of time. It would certainly be wise to discuss traffic with your driver. However, a tour leaving about nine AM and arriving at the ship at 1 PM is likely to avoid traffic problems.
Check out our trip report and pictures at http://www.bully4.us/paccoast.html
There are many agencies in LA that can arrange this for you. In our particular case we had the concierge at the hotel take care of it.
Have a GREAT cruise!