Go to the extreme south end of the lake. I can't remember the road name/number, but it's pretty much the only road that goes through there. Lake Murray State park/camping area is just off that road and completely visible (there's a big sign) from the roadway. Like TwoBit said, after going past the restrooms, turn left and go west. There's a large loop, but go straight on the dirt "road" (creep...it's full of rocks and holes). That will culminate in the dive area, so park in one of the pull ins there. "Ghost Cove" is just downhill, and it's a good place to put in.
Last weekend, water temp was about 70 degrees with vis 3-10 feet.