There are two types of underwater cameras. Those that are flooded and those that will flood. But as stated above, user error is most often the cause. Having done 3500 + dives mostly with a camera I have only had about 5 floods, only two of which resulted in a new paper weight on my desk. I have however seen many other cameras flood.
Here are my recommendations
1st - set up the camera before you go to the site or get on the boat and NEVER do it in a rush.
2nd - make sure both the O-ring and O-ring channel are clean, also do not over do it with the silicone, too much is not going to help and actually can cause trouble, the O-ring should have a wet shiny appearance without any dry areas or gobs of silicone on it. Some housings do not even need silicone on some O-rings, check your documentation!
3rd - when you first enter the water, keep an eye on your housing, most floods will show themselves quite early. If it does start to flood, try to position the housing so that the water will collect in some free space, then do a safe ascent.
4th -do not leave your camera in a community rinse bucket! I have seen many cameras survive dives and die in the rinse bucket. Mask straps, camera cords and etc. can pop open latches, also o-rings seal much better under pressure therefore even though they sealed on the dive they may actually leak in a bucket. Best advice, give it a dunk and wrap it in a wet towel. you can soak it later when you get back from the dive.
finally - get it insured!
a bit long winded sorry
