Can't answer your question about conditions in December, since the two times I have been to Provo were in June and July. I suspect it is generally the same in December though, probably water temps in the high 70's-low 80's vis of 50-100, pretty calm seas and no current. Your basic bathtub.
Dived with Caicos Adventures on the south side (French Cay and West Caico area) and with Big Blue in the Northwest Marine Park. Both were top flight operations. I think Big Blue tends to run smaller boats, but the number of divers for either operation may depend on the week you are there.
First trip was a dive only trip and we stayed at the Caribbean Paradise Inn. Nothing special, just your basic hotel with a/c at a reasonable price. Second trip this year was diving for me and my son and a spa vaca for my wife so we went to Amanyara. Total over the top luxury with a price to match. So both ends of the spectrum covered. There are many choices on Provo and lots of on-line reviews, so stick to some place that gets good write-ups and you can't go wrong.