jmax:
Which island to go to is actually part of my question!
This time of year, I'd go to Maui.
There's a lot of easy diving off Maui, go with Lahaina Divers or B&B Scuba. Molokini and the Cathedrals are the classic dives, but there's a lot of other diving also. There's also some easy shorediving there, Black Rock at the Sheraton, Airport Beach outside Lahaina are two good dives. And 1/2 dozen sites south of Kihei, although some of the entries are a little harder. You can also go on escorted shore dives with Shaka divers or Pacific dive among others.
If you do go to Maui, pick a dive op near where you're staying. Nothing further south than Lahaina if you're in the Kanaapali/Kapalua area and if you stay around Kihei, pick someone who leaves from the boatramp there. Otherwise it's a 30-45min. drive each way to the diveboat. And there's a lot of non-dive activities on Maui as well centered around Lahaina/Kaanapali. The helicopter rides are spectacular, you see parts of Maui people can't get to.
Hawaii(the big island) would also be a good choice, it's much less crowded and they have some really interesting dive sites, especially the Manta dives out of Kona. But they're generally at night, so consider that given your skill level. Kona's a nice, though quiet town. Most people stay north of there in the resorts. And see the volcanoes on a non-dive day.
Kauai's North side will be all but closed to diving due to winter surge so you'd be limited to the sites in the south. Good diving in the Poipu Beach/Sheraton Caverns area. We dove with SeaSports on the south side of Kauai. Princeville to the north will have lots of winter visitors this time of year, so the traffic between there and Lihue can be awful as it's the only major road.
I don't like Oahu, it's too crowded, so I've never dove there. Waikiki beach reminds me of Manhattan, it's all high rise hotels that block the sun. And thousands of tourists everywhere. In one day on the island we were caught in two different traffic jams trying to get to the aquarium on the east side. And it wasn't worth the trip.
I've been to all the major islands except Molokai. Next time I go back it will be to Hawaii, but Maui's a close second.