I don’t know what this structure is for. I can’t even be sure that it was built in place, although it seems unlikely to have floated in on the tide.
Could it be to prevent unwary beachcombers from falling into a beach sinkhole? (Are there even such things? [Beach sink holes, I mean, not unwary beachcombers.] Note that suspicious swirl of water near the ocean side of the structure.)
Anyway, with the early morning light on it, it was pretty, as well as pretty puzzling.