I live in San Diego...I do not think I would plan on coming to San Diego the morning of my cruise, especially in September/October. You have to drive through Camp Pendelton on the 5 freeway to get there. It is prime fire season and it often catches on fire. They suspend both rail and vehicle traffic when this happens. If there is a wreck on the 5, it could take 5+ hours to get to San Diego. It took me 4 hours to get home last July from San Clemente, which is south of Disneyland. This was just traffic related. No fires.
In the event the freeway is closed, you could always go around on the 91 to the 15 to the 8, but really, that is stressful and not a situation I would want to be in. ESPECIALLY because that cruise is known for missing its Mexican ports. If you miss the ship, you may miss a solid chunk of the cruise. Its just not worth it. Hotels are cheap that time of year. Come down late the night before, wake up and get on board.