You don't hear about transpacific cruises nearly as often as you hear about transatlantics. Perhaps this is because transatlantic cruises are a lot alike, usually ending up in London, England, while transpacifics offer a somewhat overwhelming array of final destinations. As with anything, when presented with too many options, you sometimes feel overwhelmed trying to sort them all out and simply stop considering the idea altogether.
In part, the number of days you intend to sail will determine your transpacific itinerary. If you've only got ten days, you're certainly not going to make it all the way from Los Angeles through the South Pacific Islands and on to Australia—though that is an option if you have a month to devote to your vacation aboard. Some itineraries run from California through the Pacific Northwest and Alaska to Japan, while others head south to Mexico before heading west toward Hawaii and, eventually, New Zealand.
The Pacific Ocean is the largest in the world, with more than 64 million square miles of water. You'd better bring at least a few good books if you expect to stay busy while crossing it during a transpacific cruise.
As with transatlantic trips, you should pay extra attention to the type of ship you book for a transpacific. The Pacific Ocean is, after all, the largest of the world's oceans, and it's going to take any ship a good number of days to get across it. Put your money into a boat with the best amenities and facilities you can find so that you'll have plenty to keep you busy while you're out of sight of land.