I was there in May. The diving was very poor. No fish life to speak of outside of Hol Chan Reserve. Just saw a few trained nurse sharks and groupers at the start of each dive, not much after that.
People who went to Blue Hole loved it. Good Luck, hope your experience finding sealife is better than ours.
Post a blog when you get back I would be interested in knowing if we just were there at a bad time.