Hmmm bit late to the party. Gopro housing has a small volume so any splashes when you open the box after rinsing will quickly heat up next time you use the box. Capillary action will wick some moisture to the inside of the seal. Tiny amount. My solution....wait for it....toilet paper. Good old loo roll works. Single ply or use the luxurious double ply for extra softness and absorbency. All you need is a single sheet and fold it so it cover the bottom (yes its a pun) of the box (more pun?) The tissue need to be dry obviously and will surprisingly absorb the humid air when the Gopro heats up. Change the sheet each time you open. The best thing is the tissue is ubiquitous and you could wipe of splashes when you open the box. Simple and useful.
As for why noise appears and inconsistency I think Peter explain most of it. Lack of control and also we are dealing with a tiny sensor. Now the new version is even smaller. Sensor heats up and lack of space for processing and cooling all contribute to the problem. Go Pro is a POV camera so I would not expect it to be a main videocamera for important footage. Always do tests and in doubt use a videocam that offer better features.