Long ago, back in the 60s, when I was a noobie, I "sealed" some of my first -06 ammo with the wife's clear nail polish. After a couple of seasons, I stopped that. I never take more ammo into the woods than a magazine full and never pay any attention to it getting rained on. No matter how much it rains on me, and it has a lot over those years, I've never had a failure to fire.
Strongly suspect it would take a few days of submergence at some feet of depth before any moisture could penatrate the seal of either a normally installed primer or bullet.