October - mid December except for the week of Halloween and the week of Thanksgiving tend to be slow tourist season over here - so you can often get good deals on airfare, rooms, etc..
As far as diving "normally" our rainy season is Jan-Feb. However storms can happen anytime and wreck the vis. The Water Temp started cooling off about late Late October - all Summer it had been 84, it has now dropped to 77.
Visability off-shore at spots like Molokini are nearly always great, while shore dives vary a lot depending on weather.
Hope that helps . . .
Aloha, Tim