Honestly, we don't have a real rainy "season". It's usually hit or miss when it comes to the weather. In the past two or three years or so, we've had crazy rains in January/March -- but things can change in the matter of days because we got those wonderful things called tradewinds that mean either mean beautiful days or disaster!

On a regular rainy day, we don't have the rains for more than 2, 3 days tops. Sometimes it just lasts a day. Like I said, you never know with our weather...but 85% of the year it's at least 80 some degrees outside!
I can't remember who else does interisland cruises, but I know NCL's Pride of Aloha offers 7, 10, or 11 day packages where they promise more time ashore than afloat. It only takes a few hours to get from one island to another, so you might want to take that into consideration when shopping around.
Because October-February is considered "off peak", you might also want to look into discounted air/land/car packages if you choose not to go the cruise route.