An immediate and 100% effective solution I use is to flood the GoPro housing with an inert canned gas like those used for preserving opened wine bottles. $10.
The gas in the can has no water/moisture and has a dew point well below freezing. The gases (argon/nitrogen) are also heavier than "air" so if you place the housing face down (door up) you can slowly "fill" the housing with the little tube and displace the ambient air before you close it. I still put a desiccant paper strip in the housing but there is NO moisture in it so nothing to condense.
I suspect that the "Canned Air" computer/electronic aerosol dusters would work too if they use some sort of evaporative gas. You could turn the can upside down and dispense some liquid into the housing and when it evaporates, it will displace the moist ambient air.
The little strips on their own take time to absurd the moisture in the housing so you would need to close up the housing well in advance of exposing the housing to (relatively) cold(er) water.
I got the idea from doing the same in front of an air conditioning vent.