I've used my P9 now for around 50 dives and have had no issues at all. I did a review a little while back on this camera. See
here. I have the red filter and found this to help take some better pictures in certain conditions.
You can also get an adapter to fit other wide angle lenses. See
here.
As for usage. I make sure I clean and re-grease the O-ring every few dives. Normally I might re-grease twice when diving for a week. Then like many people, when you do your check dive you can check for leaks without the camera in the housing. Put some tissue inside and see if it is damp afterwards. I'm fairly confident in the housing now so skip this and just check it in the camera bath on the boat.
Just make sure the area where the O-ring goes is clean and there is no dirt of fluff on the ring itself. Then use some silicon grease and smooth it round the ring. Then put the ring in the camera and there you go! Easy...
I tend to use the filter unless light conditions are poor. I am a warm water, high visibility diver normally so I can do this. In poor light or at night the internal flash, or preferably an external strobe is better.
Some general help on underwater photography can be found
here.
Hope this is of some help. If you need more help. PM me.