Will dive boats still go out on these days?
Boat cancellations in December are fairly uncommon. Most of them occur in January/February, if at all, even then it sort of depends on what company you are using. I cancel when its so high I'm worried about tanks/equipment/customers launching out of place, and any time I know it's a guarantee it's going to be crappy diving conditions, if it's "real borderline" in my opinion, I'll offer a cancellation (there's been times I've offered to cancel and people have said they're going no matter what... every time that's happened, the majority of them spent much of their day puking. I think this last winter I cancelled all of 2-3 charters (one being intros that I knew I wasn't going to find "pool like conditions"), turned around twice because of winds and surf combined, and had a boat full of determined pukers once. In general, if the swell's under 10 feet or so at the harbor mouth you can find some decent dives somewhere, if it's breaking across the harbor mouth you might as well forget it most of the time (angle the surf is coming in can be important).
If you're looking at shore diving, pay attention to the evening surf news. It's generally for Oahu. If they say a big northwest swell is coming in "tomorrow", it'll probably be clear in Kona 'til the following day (it generally takes 12-24 hours longer to get there). In the winter, a 10-15 foot swell (based on Oahu reports, not actual Kona conditions, Kauai/Oahu/Maui actually shade the west side of the Big Island from the bulk of the swell) often amounts to nothing in Kona, a 15-25 foot (Oahu) swell can be easily worked around in Kona by picking spots shaded from the northwest, 25 and up (Oahu numbers again) might mess things up pretty good, but might not.
In Kona, if the swell's under 10-12 feet from the northwest you can usually find a spot to dive. From the direct west, swells of smaller size can mess up most of the coastline, that's usually happening in early spring or late fall if it happens at all.
Just plan on looking at the shore diving day to day, you'll probably be fine, if it is crummy it usually only lasts 1-3 days that time of year. Odds are it'll be a non-issue for you. I hope this helps some.