I've a Casio Waveceptor WV-M120. I had it for a couple years. Rated 10 BAR and I could do anything with it before like swimming. 6 months ago, I got fogging inside. I popped off the lid and dried everything in a desiccator. Put it back together and it was ok, then it started fogging again.
Rear case groove, the back cover surface and o-ring all look to be in good shape. How likely is it that the leak is from the button stems? O-ring aging and becoming permeable to water vapor?
Desiccant charge inside watch that's full?