No idea how long this has been here, found it by accident while visiting the boat ramp. Cars/trucks were parked in the grass and loads of people on the shore and playing in the water. Short walk from parking to the shore. All sorts of boats were in the nearby area. It's not the most refined launch spot (slight dropoff) but it should be possible to launch a personal watercraft with some effort.
NOTE: According to the website at http://www.lake-ray-hubbard.com/elgin-b-robertson-park-south-ramps/ "Park now closed and being developed commercially by the City of Rowlett." Don't know how long that notice has been posted or if it's actually in effect.