There are some cruises that leave out of Long Beach ... but they usually take you to Ensenada, Puerto Vallarta, Cabo San Lucas ... but you don't spend much time in the cities, its more about spending time on the boat and then stopping off in the cities.
Not sure what you want to do in Mexico City?
I'm quite curious as to why you're going to Mexico City. You could go there, but it's certainly not the first choice (or even the top 10) for a tourist area. It is a very large (and polluted) city. Cabo San Lucas, Puerto Vallarta, Mazatlan, Acapulco, and Ixtapa are certainly better choices for a vacation.
That aside, you're only viable option is to fly. I'd suggest flying from LAX as you have many more choices and can get a non-stop flight. You can fly on United, Delta, Alaska, American, Mexicana, or AeroMexico non-stop from LAX. If you fly out of John Wayne (Orange County), you'll need at least one stop-over. Look at Mexicana's websites for "Mex-E-Savers." It looks like you can fly RT for $259 plus tax.
Great advice CR-about the flights!
You can no more take a cruise to Mexico City than you can take one to Ayer's Rock.
Flying is it. You can fly out of Orange County to Mexico City, but you'll have to change planes in Dallas and you'll pay a premium price.
May I suggest you buy a map of North America to help you plan your trip?
Fly from Lax as for Mexico City being polluted, big, dirty and crowded yes I'll have to agree but I can guarentee you that it is still worth going. It is a city full of beauty and culture that never sleeps and remember that especially if you are visiting the pyramids at teotiahuacan which are a must. So would I live in Mexico City... NO but I would go and visit it again.... in a heartbeat.