Hi JustJules,
Water temp should be 83-84 degrees, and it's very unlikely to be below 82. Coz really has no thermoclines.
Air temps should be daytime highs in mid to upper 80's & nighttime lows in mid-70's.
Modest thermal protection both u/w & at night should keep you comfortable.
Sep is the very beginning of rainy season, so there may be a few passing showers, mostly afternoon & evening, but it usually doesn't disrupt diving.
Now for the kicker--Sep is nearing peak tropical storm/hurricane season in the Gulf/Caribbean, and if one hits all bets are off.
Best of luck.
DocVikingo