I'm presuming they are the maritime version of this: -
Friday, February 19, 2021
Did you ever hear of a 'special mark' in relation to maritime navigation? Me neither. I've heard of a wreck buoy to mark a wreck; cardinal marks and lateral marks but this is a new one on me.
According to the CIL website Special Marks are used to indicate a special area or feature whose nature may be apparent from reference to a chart or other nautical publication. Special marks are yellow. They may carry a yellow 'X' top-mark, and any light used is also yellow.
My thanks again to Nick Blackburne from Holywood who spotted this one just off the shore from his home town, obviously marking the location of a gas line. I've no idea when they were introduced.