I spent Christmas and new years in Kona a couple years ago. I used Jack's, I recommend them. All their boats are more fishing/sport boats (Aluminum or fiberglass hulls). The big boats have like 24 divers, the smallest boat (I was only on it for the Pelagic dive) has like 8.
We got blown out for two days in the two weeks we were there, that isn't uncommon in winter. One of those when when a bunch of family was gong to do the manta snorkel, so you might want to try to schedule it at least twice just in case. The manta dive is very cool, don't miss it.
There are limited sites that they use in winter due to the weather patterns. If you have a reasonable amount of experience and AOW they run advanced trips that hit some more complex sites, but for a few days you won't run out of places to see on the basic trips.
In winter there are a lot of humpback whales. You will probably never ever see them under water (among other things, federal law says you can't dive if they are there - they have to show up while you are already in the water), but you probably will on the boat to/from the dive site. You might hear them under water too.
Can't talk about the rest of the islands.
Boat diving in Hawaii isn't cheap, but it's cool.
