When Bet was a child, her father tried to teach her to body surf, but she was unable to learn to ride the waves, she just stood there, letting the waves pound her, "as if standing staunch were a virtue".  This is similar to the way she deals with life.  Bet's way of dealing with the troubles of life is to stand firm and endure.

Bet's life, like most ordinary lives, is beset with problems.  Her husband has left her, and she has a mentally handicapped son whom she must put into an institution because he has become too much for her to handle.  Bet's life is like "unprotected waters" where "average waves" of trouble come one after the other to pommel her as she stands steadfast and alone.

The title reflects the image of a sea with waves rolling through consistently at intervals.  The word "average" indicates that the waves are not too large nor too small, and as a metaphor seems to imply the challenges that are common to most normal lives.  Beth's life, though difficult, is not unusual.  Life in general is like being in "unprotected waters", and buffeting waves are an expected part of the experience.