The marker shows the approximate location of the hulk HMS Tamar after she was scuttled on the 18th December 1941. From then until the wreck was cleared (c.1947), Tamar's three masts are visible, sticking out of the sea off Wanchai. You can see them in this photo by Hedda Morrison: find the tall chimney in the RN Dockyard, extend a line upwards til you hit Kellett Island, and the masts are just to the right of that line.
We're looking for any photos that show the masts, but especially photos taken between August 1945 and March 1946. Stephen is hoping to see a flag raised there, which would show the wreck was still being used to represent Hong Kong's "HMS Tamar" naval base. There's more about that possible flag in the comments at page https://gwulo.com/comment/49777#comment-49777, and more about the history of HMS Tamar at https://www.scmp.com/magazines/post-magazine/long-reads/article/2002369/all-about-ship-gave-hong-kongs-tamar-complex-its