To add to what Dark wrote:
The Cooper meanders through what is basically a peat bog on its way to Charleston harbor. The river channel is a large U shape with the walls made up of the "peat" and the river bottom made of marl. The water erodes away the walls of the channel and dumps fossils into the river bed and they get washed down stream and settle in low areas along with rocks.
The low viz that Dark is referring to is due to tannin in the water caused by rotten plant matter. He's not exaggerating about the 10 - 12" visibility.
The currents can be very interesting. We used to try to dive at slack tide but that wasn't always possible. We used to weight ourselves a little heavy and get to the bottom as quick as possible. The quicker you're laying prone on the river bottom the better off you are. I used to carry a small crowbar to stab the marl bottom with to help pull myself along looking for that sweet spot.
When my bottom time was up I'd stand on the bottom leaning into the current, adjust my gear, listen for boat traffic (the stinkin jet skis were the most hazardous as they tended to ignore the dive flag) and jet to the surface as fast as possible blowing bubbles all the way. The boat captain spent the dive positioned up current watching our bubbles. When a diver broke the surface the boat would slide alongside and pick them up.
It's not your typical buddy diving. You can't keep up with a dive buddy in those conditions. Great fun but you need to be confident with your skills and your gear.