I do agree DFC. One of my reasons for asking the question is that one of my favorite things to do is to repeat something that I did in my training.
Having learnt 30deg turns, 60deg turns, climbing turns, descending turns etc my instructor asked me to fly above a sea wall at 200ft and keep the wall below me, with a +/- limit on altitude of 20ft. After 5 miles, we turned around and did it again at 100ft.
That being OK, we did it again at 50ft.
He did the final run, touching the wheels on the straight bits and lifting up a bit occasionally, to clean the fences and gates that sat across the wall.
I've repeated this many times myself. Its a wonderful rush and at the location in question, is safe being next to vast reaches of open fields and devoid from walkers. Its excellent practice for getting into a small airfield in challenging gusty cross wind conditions as you learn to put in considerable turns with perfect altitude control.
My concern recently was that a sea wall could be construed a structure.
I always do a flypast to check for walkers or animals and am now happy to continue doing this on the definition that people cannot inhabit a sea wall.