There are a couple of groups who regularly go out to Red Top Mountain State Park for open water swims:

1) Swim With Pete:

Probably the most “famous” of all groups.
September swims are Saturday the 10th and 17th (they usually pick two Saturdays per month).  Arrive at the lodge  by 8am so all can be signed in and swimming by 8:30am.  First time folks need to sign a waiver.  This group has volunteers in kayaks (how awesome is that?!) for safety – bring a few bucks to tip the kayakers.  If you know any kayakers please spread the word – they are always looking for volunteers.
For more info e-mail swim.with.pete@gmail.com

2) Atlanta Tri Geeks:

Fox’s Tri Geeks have a ton of fun every Wednesday.  They start with an 8am run at the Brewery, then ride the BBC-26 and cap it off with a road trip to swim at Red Top followed by a picnic lunch.  You can reach Fox at: trifox<at>bellsouth.net

General info:

Where: Red Top Mountain State Park
Emerson, Georgia
(770) 975-0055

All groups tend to swim in the same location: a small cove to the right of the lodge (the lodge is now closed but it does still have a small store that sells snacks).  Just walk down the trail immediately to the right of the lodge called “Lakeside Trail” toward the cove – it is marked with a small wooden sign at the start of the trail.  You should see the water within a dozen yards (or you’re on the wrong trail).

If you swim from deep in the cove out past the metal boat dock to the rock outcropping it is about a half a mile (800m).

See also: http://www.gastateparks.org/RedTopMountain
FYI: Water is very warm this time of year so bring something to drink and a snack