thanks for the comments ... Susan, I'm nto sure what it is. It is usually underwater when the tide is in and is right on the edge of a harbour entrance so I reckonit's quite dangerous - I cant imagine it was a mooring point as it is on the rocks. it's weird, that's for sure.
Jon, strangely enough, the only shot here with an ND filter is the middle one, and that was only 3 stop one. the other 2 were taken at between 5 and 6 in the morning before the sun had risen hence the long exposures.