Your concerns are real. I used a hard core marine spar varnish on a hickory bow I used in Missouri. It held its draw weight. Spray-on spar urethane, Helmsman brand, has gone on some bows, over tung oil or Tru-oil (clear satin finish helps reduce sheen). This is not primitive, but neither are seasons, buck or doe restrictions, etc...
My first bow was hickory. Designed at 44# @ 27", I found the monsoonal Arizona moisture and humidity dropped it into the 38,39 # range when coupled with the fact with my actual hunting draw is 26" (and the set it took). I missed a lot that backcountry season. Shot low, a lot; my friends still abuse me over that summer.
I "dry out" my bows above the campfire or camp stove when hunting in wetter conditions, just enough to get warm-to-touch.