Thanks for the ideas.
I had used the boat in salt water, and the wing nuts were crusty and difficult to remove. Then I canibalized those parts to put in another boat, ordering a replacement for the Wenona. Those new parts had smooth threading bolts and nuts, darn it! I only had them in for two uses before one of the wing nuts decided to fly away.
I will look for the stop-nut version of wing nuts. I like being able to move the foot brace and seat to adjust the trim of the boat, so I'm reluctant to use locknut. I may have a hard time finding elastic stop nuts in a wing nut this small, and if that turns out to be the case I will look for the non-permanent version of locknut, or form-a-gasket, per GBG's suggestions.
Come to think of it, just a plain old lock washer might help, and I probably have some I can put to immediate use.