Wrong! An E-5 (long ago) by the name of Les Bowers set me straight. He served in Vietnam in 1968 and 1969. Before that, he had been in Thailand for a short tour. He was assigned to the 165th Light Amphibious Co (LARC V) at Wunder Beach during the summer of 1968.
The stuff I thought might be concertina wire was actually anti-submarine netting left over from WW II. It was brought off shore by the AB&T Barge Company and unloaded to Wunder Beach by the 165th Amphib Co. During the off loading exercise one of the LARCs had a load shift and almost capsized. The load went overboard, the LARC righted itself and 10,000 lbs. of netting went to the bottom, never to be recovered.
The plan was to build a breakwater with piles of the netting becoming the base for the jetty. The project was never finished (at least by 1970) and the netting remained as part of another Vietnam plan never completed.