I spent a few days there in January. Hotel was fine, good food (we were all inclusive). There were lots of Russians there.
The dive centres will be open every day. Dunno about Sharm, but the ones in Dahab had a late start on Christmas day and only did one dive instead of 2.
I wasn't too impressed with the Shamandura, but I only did one day's diving and it was too much hassle to arrange diving with someone else. It's a 20 minute dive into Naama Bay where most of the dive centres are located, but they'll arrange pick up/drop off.
Umbi Divers are just around the corner in Sharks Bay, I've heard good reports about them.
This was the view from our room, you can just see Tiran in the distance. It was raining when I took this
