I fish with my GoPros on my kayak all the time. In Costa Rica I noticed that they tended to fog on me. I always try to put them in the case with all of the gear at outside air temp. But I was still getting fog. It turns out the camera itself was causing it. I could fish all day in the sun and not get a hint of fog in the housing. As soon as I would turn the camera on to film a fish fight, it would only take about 3-5 minutes to fog the lens cover, even though the camera had been in the same location on the kayak for hours. Something about the extra heat from the camera inside the housing. I've modded my housings to try to get better sound anyway, so now I don't have that problem. Housings are cheap. Drill a small hole in the housing and try to keep it in the burble. You can always tape the hole if you like. Just some food for thought.
PS. One thing I've always wanted to test was if I left the housing outside to warm up but left the camera in the air conditioned hotel room. Maybe the air inside the housing would be dryer...