Hi Elena:
My wife and I are on the PG the week before you two. Masaki is correct: if possible, take your academic test prior to departure. I recently went through the full PADI certification in Seattle because I was originally certified in the mid-1960's, and it took four days total in a one on one class. The cost was $350 for instruction and gear rental.
You purchase the PADI open water course (book, dive table), read the material, and take a 4 or 4.5 hour test. It is pretty easy: there is a test on each chapter and a final, but you need to spend a few hours reading and completing the book questions and exercises. There are a couple of pool dives that take a half a day total, depending on how you do.
There are then four open water dives, which you can either complete in two days (two diver per day for two days), or get a referral to take on the PG. It would be very easy to complete the academics and pool dives in southern California, and have more time to enjoy the cruise.
By way of information, if you bring your own regulator I received a posting from the ship that said they use yoke stage 1 valves, not DIN.
Hope this helps,
Dan