Wilko wrote:

A couple of years ago I had difficulties rescuing a friend of ours who
was getting recirculated, I remember that I was amazed at how long she
stayed under during each circulation, despite wearing a PFD.

In desperate circumstances, you're better off taking off the PFD and
diving down to follow the bottom current out of the backwash. Few
people who get in (low-head-dam) trouble have the knowledge or composure
to make that radical move, however.