Water temps in the "local quarry" will be really warm then. It's 76f right now on the bottom.. In sept it should be up to 80f or more the way it's going. Typically in Ocotber the Madison Quarry temps are still in the 70's. It's a great time to dive out there.
The Gulf is in the 70's also come October. Not tropical, but still nice and warm. We went to the beach (and dove) last year in October and it was perfect weather.
Rentals aren't a problem. There are two full service dive shops in the area
SDI Scuba
http://www.sdiscuba.com
Dive Sports
http://www.divesports.com (Dive Sports even posts on SB and supports members here).
Both of them rent gear for about $25 bucks a day for reg and BC. Tanks run $6 to $8 with first fill included. A wetsuit averages about $10 bucks. Dive Sports has a "Quarry Days" package for a BC, Reg, and Wetsuit all for $25 bucks. Both Dive Sports and SDI have good compressor systems for clean air.
As PhilmontTrek mentioned, we've got the SEAS Club, which is the local scuba club. It meets on 2nd Thursday of each month. Looks like you'll be here for the Sept and Oct meetings. You are welcome to attend. It's open to anyone. It's a lot of fun and is mostly just a local diver get-together that is about diving. We'll be doing a 'grill out' and most likely a night dive at the local quarry sometime in that time frame.
Of course if you stay the last weekend you are here, you'll be here for
Vortoberfest in the FL Panhandle!.
So... believe it or not, this area has a pretty active dive crowd. lots of folks from this area on SB also.