I did my openwater dives in Lake Ouachita outside of Hot Springs in late October. Since I didn't know any better, the diving was nice and the visibility was fairly good (about 12-15 feet). The surface air temp was 55 and the upper level water temp was about 65, although it dropped pretty dramatically at the thermocline. It was very comfortable with 10 mm of wetsuit on. At the dam, there are two boats, a four-wheeler, a kayak, a mailbox and a matching toilet/sink set sunk. One of the boats is at about 45'. The fish are fed, so they're quite friendly to the divers. The only bad thing about diving the dam without a boat is the very long staircase you have to descend to get to the water. I'm sure it would get tiring with lots of gear.
If you ever want to go diving, PLEASE, PLEASE get in touch with me!!! I'm in Texarkana (southwest corner) but I'll go almost anywhere to dive, as I'm new and very enthusiastic!