I was wearing a skimpy kevlar helmet that "blew back" and so I struck a
couple rocks dead on with my forehead. Due to a poor design that only
had a single stap on each side buckling in the center. I've found that
helmet strapping systems like the one Grateful Heads uses with 2 straps
on each side coming to a V below the ear and then buckling under the
chin can completely eliminate this problem.

I've hit my head hard enough with a GH helmet to "see stars" and cause
stress (but not breakage) to the helmet (which was retired and
replaced), but that didn't cause a concussion.

And you're right - when I was an open boater, I never had any problems
at all with head injuries.