I have been on a trip to Salt Cay and one to South Caicos. I did my diving in August, so I could not say about water temp in December.
South Caicos has very good diving. Both Salt Cay and South Caicos have local fishing pressure so you will not be seeing too many large grouper. I have heard that they also hunt turtles. I saw some turtles but they took one look and took off.
I preferred Salt Cay. Ollie and Debbie run a very good dive operation and both were as nice and as helpful as they could be.
The visibility was quite good. Both places have good walls. Also both places have very good corals.
On top side, South Caicos has virtually no tourist infrastructure. It does not even have an organized restaurant. One really does not seem to mix with the locals on South Caicos.
The whole thing was very different on Salt Cay. About 70 people live on the island. I think I met over half of the population in one week. Everyone was quite friendly. There are 3 or 4 restaurants on the island. But their stocks are limited and usually frozen. So if you are coming for dinner, you often have to order your meal at about noon so they can thaw out what they need.
Air Turks and Caicos is indifferent to schedules. The day we went, they were supposed to fly us in from Grand Turk. But they could not decide if they were actually going to do it that day. So we managed to get a ride on the once per week ferry from Grand Turk to Salt Cay. "Ferry" is perhaps over stating it a bit. It was just your basic boat and it was loaded with people and cardboard crates (most of which seemed to be soft drinks). We chatted with people on the way over. I think that if you have problems getting over from Grand Turk, Debbie and Ollie can come over and fetch you.
The description of Salt Cay as the "island that time forgot" is pretty valid. It has its small community of expats and belongers who are trying to keep a viable community going. It seems that anyone who is trying to keep things going there is readily accepted. A problem that the locals have is that their children leave for other places: USA, Canada, Britain, etc because opportunities just are not on the island.
Also things are a bit basic on the island. There are donkeys, cattle and chickens wandering around. I think most of them are wild. The cattle look like if they were slaughtered all they would yield is raw hide chew toys. The island itself is dominated by the salinas: those are salt pans that were used to yield sea salt. Salt Cay used to export sea salt in the 19th century.
Salt Cay is not for everyone but the diving is quite good and the little community is fascinating.
