Definitely early September. The breezes are still from the east and are warm and don't usually get more than 15 knots but November brings cold fronts that shift the wind to the north northwest at up to 25 to 30 knots which makes diving conditions much less favorable. If you get lucky in November, you hit Belize when no coldfronts are near and you have a great time. If you are unlucky, you have 25 to 30 knot winds for five days straight with grey skies and drizzly rains. It can rain in September but squalls generally pass the cayes quickly with the trade winds. Also, since October is so rainy, visibility in November can be much worse than September. This last year November got us down to thirty-forty feet vis here in the south (about an hour boat ride from Turneffe elbow) and it took till mid December to get back to sixty feet and now it is well over that.