Hi W. D. Nice to meet you down at the bay a week or two ago. I think the black marker off Treasure Island that was discussed is the one that is off the east or southeast side of the island and is a fair distance from the island. I think you dove on the shoal that is between Treasure and Rattlesnake, and east of Sleepers. That black is pretty self explanatory, and has the red buoy at the other end of the shoal along with the flashing light.
The black buoy "on the other side of the island" is tough to spot at night, and is not one of Bizer's waypoints on his download file, so I finally remembered to ease on up to that rascal during the day and make my own waypoint. The others are right, if you are on an east west course, and approach that marker without looking at your chart, it would appear that you should pass north of the marker which would put you between the marker and the southeast end of Treasure. I fear that would mean "can you spell lower unit gone!!!!!" (or keel in the rocks!!!!)
Bottom line, GPS is great, but RTC or RTM, otherwise known as Read The Chart or Read The Map.
BTW, diving that shoal north of Treasure ought to be like trying to dive on the Lady. Boats just fly through that shoal area, on plane, and really don't care how close they are to you. We have anchored out there and had double digit number of boats pass within 50-75 feet on plane, almost like we weren't there. Not to worry, we have been there three times, and never more: it was the first, last, and only time.
Dive safe, and hope to see you before winter sets in.