To answer the unanswered portion of your post:
I just came back from Key Largo about a month ago - arrived there with a fresh c card (not ONE dive on it outside of the class). The best dives of the whole trip were actually through a company on Islamorada, Key Dives. Our guides were awesome (they put 2 in the water for the 6-8 of us diving), and the whole experience was just amazing. Nice, friendly staff, clean boats, great sites. If / when I get back down there, I won't dive with anyone else. It's about a 20 minute drive from Largo, depending on where you are, but totally worth it. One cool feature was that one of the guides carried a slate so she could tell you what you were looking at in real time - also handy when we came across a scorpion fish. Their prices were good, too! As to water temps, I can't speak for December, but when I dove 10/30/11, I wore no neoprene at all on a two tank trip to 65 and 60 feet and was comfortable the whole time.