Good water Camera But not when humidity strikes

We went on a river float trip this year. Myself and 2 of my friends had all purchased this camera to document our ride.
The camera is waterproof, however, all 3 of us had the same problem--about halfway through the roll of film, the camera viewer (plastic outer protection) began to fog up...so on all 3 rolls of film, the first half comes out clear and the 2nd half of the exposures are all very foggy and hard to see.

I guess it was just the humidity on the river or something..but I have used these before (even on the same river) and have never experienced this before...