About 10 years ago the CAA , for clarification(?), proclaimed that a runway IS a structure for the purposes of Rule 5! Unfortunately I can no longer find reference to this, and I can't remember the exceptions, but they were quite pointedly saying that you can not fly low over a runway!
For those of you worrying about take-off and landing, that is specifically exepmted in rule 5 and I think that intentional low approaches are permitted at licenced aerodromes, but if you were to fly low over Popham's uninhabitted, unlicenced grass strip or do a low run & break after hours at your home aerodrome, then you could be prossecuted under Rule 5.
As for sea walls, well they are clearly structures and that blatant abuse of the rules would get you instantly drawn over hot coals if the CAA ever found out. It sounds marvelous fun when you get away with it though!!