My flood occurred in Bonaire after diving in very clouded water resulting from an unusual wind shift causing waves to pound the leeward side. Visibility near shore was under 18"; along the reef, only closeup photos were possible and those had to be done without a flash because of particulates. Returning after the shore dives brought us through the very low vis near shore. Two days later, on the 2nd to last dive of the trip, my camera began to flood about 1/4 through the dive. Showed up as condensation, that spread, with the droplets becoming bigger. Couldn't do anything except continue the dive. Camera functioned throughout. When I surfaced and returned to the boat, there was about an ounce of liquid inside the case. It has penetrated the battery compartment and there were droplets on the inside of the LCD screen. Camera never revived.
Postmortem: the o-ring on the case was covered with very, very fine particles of sand that were embedded in the silicone grease. I use a very thin layer, just enough to cover the whole surface, so only the bottom part of each particle was actually in the silicone.
What probably happened: when I opened the case to upload the images from the memory card on the days after the cloudy dive, I probably caused a slight rotation of the o-ring or the material on it. That slight rotation was probably enough to transfer particulates from a benign location on the o-ring to a point between the o-ring and the camera case, causing a slow leak.
Conclusion #1: This slow leak allowed a small, but lethal, amount of water to enter. The water could have been rendered harmless if an absorbant pad had been in place to soak it up.
Conclusion #2: I was lazy and should have attended to the possibility of fine particulates getting into the nooks and crannies; I will be more attentive in the future. Q-tips are very helpful for cleaning the groove in which the o-ring sits.