There don't seem to be any visiting boats recently! Easy ... head for Penarth pier (on transit from Flatholm) then towards barrage(well marked channel)....call barrage control on 18 then go into the bay, cbyc,cyc, marina or mermaid quay.
In the first leg up the Wrach Channel from the Outer Wrach Buoy to the branch off to the barrage you can use the Sectored light over towards Cardiff Docks (Its on and still easy to see during daylight hours . .Cant remember the exact timing but its the usual red green and white sectors.Stay in the white and your OK until you branch off before the preferred channel bouy.
The red can buoys sit aground on low water springs so keep them about 30 to 50 ft to port as you come in.
This picture (borrowed from CBYC gallery)is taken from the Outer harbour entrance looking back out the entrance channel at the bottom of a very low spring tide.
The preferred channel buoy is to the left of the picture.