Disney seems to be able to fill up their 3 cruises a week year round with little problem. Probably a "supply and demand" thing. That may be why they are thinking long and hard about IF they want to add ships (and passenger capacity).
My mom takes off next Sunday on Celebrity's Mercury for 7 days, San Francisco to Monterey, to Santa Cruise to Cabo and back to San Francisco for $389 for an inside cabin. Not the warmest time to cruise, but for her, the ship is the vacation, the ports and just an added bonus.
She did a repositioning cruise last year, 7 days around Hawaii, then on to Vancouver for a total of 11 days at a very good rate.
She did splurge the year before and do 14 days around the tip of South America, that was more expensive though.