Karen, I spent 21 months in Hawaii (Oahu) some years ago. Unless the weather has changed a lot, December is fine. We did have a lot of rain during the two Januarys I was there.
The north shore of the Island is where the surfers have great fun during the winter months. Thirty foot waves are awesome to behold. That kind of action depends on the weather in places other than the islands.
We are cruising on the Radiance of the seas next May and I can't wait to relive some old adventures, however people tell me I won't recognize the place.
Have a good trip and don't worry about the weather on the islands, I have never seen a place where the weather is as consistantly nice as the Islands.
I cruised from Honolulu to Oakland courtesy of Uncle Sam's fine gray cruise line. We sailed in March and the sailing was PERFECT, the weather anyway... it was not a gourmet cruise by any means.
Dave & Susan