What's the best way to protect a board from corrosion in a hot/moist environment?
Sounds like your boards are in a very tough environment, but anyway, it is common practise (and an accepted industry standard) to use so-called Conformal Coating.

I know from experience at one of my previous companies that it does help.

Material varies, but the article in the above link explains a lot and internet searches for "Conformal Coating" will yield many products. I have used brushes to apply the coating after reworking PCBs, but in volume production, spray or dipping are used.