I've been diving the Red Sea many times.
There is no specific season for diving the Red Sea, but, as mentioned already, in December their might be a lot of wind which cause waves as well.
For the Sinai peninsula you should go for some other time, because often the water is warmer as the cold winds from the mountains around (sometimes there is snow on the mountains). Anyhow Sinai pensisula is pretty well developed and boat goes out daily to the reefs such as Ras Mohammed (Shark & Yolanda reef), but not for the Blue hole close to Dahab (about 1h by car).
But you might go for Jackson, Thomas, Woodhouse reefs in the Strait of Acabar as well.
All above possible by boat day trips based in Sharm el Sheik area.
The Thisslegorm is only accessible by liveaboard trip (about 3-4h by boat one way from Sharm).
Now more to the south:
The Brother Islands are only accessible by Liveaboard trips as well. Tiny little islands without any vegetation at all, but beautiful underwaterlife, chance for pelagic fishlife and 2 outstanding wracks (the Aida and the Numidia)
Elphinestone might be done as day trip diving from Marsa Alam, but I don't recomment to do so.
Both Brothers as well as Elphinestone reefs are in the open water with some hundred (in fact it is more than two thousand) feed of open water straight down next to the reefs. There is always current around, which might be strong.
Same with Daedalus reef, which is even more far out.
In general in winter there is not that many fish in the north of the Red Sea (Egyptian Red Sea) and, as mentiond, conditions might by rough.
I also recommend to take at least a 7mm Wetsuit for December.
But best saison for diving is May to November.
Visibility is always great in the Red Sea
I really recomment to run the diving by lifeaboard trips:
- The North Egyptian Red Sea you migh do with the Thistlegorm and Ras Moammed. Don't miss the wracks at Abu Nuhas.
- If you would go for the Brothers, Daedalus and Elphinestone take a second lifeaboard trip starting either in Marsa Alam or Hurghada. Better don't stay in Hurgharda landbased.
Have a great trip
Quite frankly - Red Sea is a place where you dive the open waters and it happend from time to time that divers got lost.
I really recomment such a liveaboard trip when
a) you are an experienced diver (judge yourself what you consider experienced - I did not dare to go there myself for many years)
b) Condition are good