We just returned from 10-day Hawaii/Ensenada on Celebrity's new Infinity. It was fabulous.
You absolutely MUST schedule your trip to coincide with the whales! We just caught the end of the season on our April 18 cruise, I understand. They were headed back to Alaska.
We had a cabin with verandah and were able to spend 2 days in Maui watching the whales romp right off of the ship! It was spectacular!
I don't know what's the best time for prices - only that if you want to see the whales it's something like January - April.
We had perfect weather the entire time - we spent 3 days pre-cruise on Waukiki at the Hyatt Regency then boarded the ship in Honolulu.
We were concerned about the 4-days on the Pacific back to Ensenada - but the Pacific was smooth as glass. And we used our verandah all but 1 day.
Enjoy it what ever you do!