What a great question, I always wondered but never thought to ask.
I wear prescription sunglasses and once they get wet or salty they're almost useless, but better than no glasses.
Years ago I had contacts but they also have issues. And you still need sun glasses.
If you are near a SAMS or Costco you can get a dup set made from your current glasses, I had a spare pair made for about $80. They are now my primary set.
Never tried wax but this reminds me of stuff we used to put on our dive masks, maybe that will work, I'll be checking that out soon.
We could also find out who makes those tear away windshields for NASCAR and have them make some little ones for our glasses.