Haven't got my log book at hand, but if I recall correctly, water temperatures during the winter are in the mid 70s. I don't know how easily you get cold - these things vary so much from person to person, of course - but I'm usually toasty in a 5mm. However, I know folks that wear 7mm suits (plus additional protection via a hooded vest) in winter, especially dive professionals that make multiple dives a day for consecutive days. I'm sure that if you posed this question on the Hawaii Ohana section of SB, you'd get lots of different answers!
If you're coming to Oahu in December, your shore diving options will be limited to the south and leeward (west) sides of the island. You'd get ground to a pulp if you went to the north shore at that time of year, but the west side is pretty calm year-round, and the south side is at its best in the winter, and fairly accessible...I made my first south side shore dive of the season about a month ago. Some south side shore-diving sites involve a bit of a scramble to and from the water, so I wouldn't recommend a drysuit, even with reinforced knees. Moreover, you'll be sweating like a horse on the way down to the water - it's still pretty warm topside! - which is another argument in favor of diving wet.
As gorgeous as the south side shore dives are, entrance and exits can be a bit tricky at some places, so I really recommend you dive with a guide. Do a quick search of the Hawaii Ohana section of SB, and you'll find a few names. Quite a few of of them are active SB posters, so you can private message them directly, if you wish.
Have a lovely trip. You're coming at a great time - the humpback whales will be here in full force, so expect your dives to be accompanied by whale song.
