I went in December over Christmas. Cannot recommend it highly enough. It was certainly some of the best Caribbean diving I have seen. I was on the Jardines Agressor II, an incredibly great boat and crew. Several Aggressor veterans on the trip said it was by far the best Aggressor boat they had been on.
From what the crew said, winter (Nov. to March) is generally the best. The weather is usually cooler and nicer, fewer bugs. Winds are generally from the north, which is good because the reefs are close to shore to the south of barrier islands, and so pretty sheltered. Also, summer is the rainy season, which can affect the visibility a little due to runoff from shore, and also the clouds can block direct sunlight from reaching the reefs.
On my december trip, the weather was completely sunny, air temps about 80 F. Cooler at night. Water temps about the same. A little breezy but the reefs are close to shore and sheltered from northerly breezes in the winter. It was faultless.
I would go now, in the next month, or maybe wait until November or December.
Thanks, I have to wait until November or December,