Early December is the middle of the
Triple Crown of Surfing, so expect good size swells out of the North to Northwest. Northwest swells mess up much of Maui's shore diving (Oahu same to worse). Most dive sites react differently depending on angle and size of swell, making predicting which dive sites will be best very tenuous.
Whichever side of Maui you choose (West or South) boat diving or driving to shore sites might be neccessary (due to swell). If on West Side, Lanai is the boat diving, minimum 45 minute possibly rough channel crossings required, and shore dive sites might be all the way down in Makena (South). If on South Side, Molokini is only 17 minutes from the ramp so your done by 10:30AM and Makena is even closer.
The West Side has more night life and dinning options but the surf contest (Dec 8 - 20) will congest the limited roads. The South Side is more expensive but has much better roads. Whale sharks are seen at Molokini occassionally (Nov. ?). There may be a 3 tank Lanai trip with 3rd tank south of Lahaina at Manta Way. Manta Way is also possible with a few of the Kihei Opp's on a
very limited basis, but might not be done until mid winter.
South Oahu is known for it's wreck diving, they are usually good that time of year and if you wanted to brave the traffic the surf contests are entertaining to some. There's lots of people on Oahu so It's not paradise to many, but the entertainment and dinning options are impressive.
My Big Island (Kona) diving was all from boat, late November '04. There were Northwest swells, but lack of soil means visibility stays good near shore (youngest island). Strong swell for my night manta dive tossed and tumbled those not holding tight to the rocks, but it was one of my most thrilling dives ever. Need to go back for the 3-tank Au Au Crater trip. Day trip to the Volcano is a long day, would stay at Volcano House next time.
I would only dive Kauai in summer (Tunnels / Ni'ihau), but there is plenty of winter diving in the Poipu area (South), as well as shorefront accomodations. Kauai is a much quieter Island with magnificent physical beauty (Waimea Canyon / Napali Coast - lots of trails) and great golf.
The Hawaiian Islands are rarely a dive only vacation and I suggest keeping non-diving interests in the mix. Mother Nature does occasionally put the kibosh on diving.