Here's a link to a review of your camera, it may be more clear than the instruction manual:
http://www.imaging-resource.com/PRODS/P73/P73A.HTM
I have used the P-5 and P-10. The P-5 was last seen off Witche's Hut at Bonaire.
Don't worry about the buttons. The 40m depth limit means that the buttons will be fully compressed, so they won't function. It doesn't mean the camera case will implode. Soaking the camera in clean fresh water for 20 minutes or so after diving will disolve salt which works its way into the mechanisms.
Be very careful with your "O"ring. Don't stretch it, don't scratch it, grease it very lightly, don't let dirt or excess silicone build up in the "O" ring channel, don't get any lint, hair or tissue flecks on the ring. Always check the "O" ring through the clear camera housing to make sure the silicone seal is patent and there are no fibers on it. The smallest thing interrupting that grease seal will provide a path for water.
Don't leave the camera on constantly as this can cause fogging problems. Load the camera in the dryest environment possible. An air conditioned room is good. Small humidity absorbing gel packs can help too, but watch your controls.
You will probably want to turn off your AF illuminator. I think the lens bezel on the housing will block it.
Auto focusing. Setting it to monitor can help with a moving subject like a fish, but it uses more battery power. Remember to slightly pause as you set the button halfway down to let the focusing process occur.
Spot mtering helps alot with exposing for your subject, but you have to keep the spot on the subject to meter properly.
I usually set the ISO to 200 and use "P" (programmed mode). I have also found using fluorescent light white balance helps to get some red into U/W shots without a flash. I use the macro mode most of the time. The flash is only good for less than a meter, but it can throw a nice highlght on silvery fish in very clear water.
I have a Sea&Sea YS 25 flash and their wide angle lens. That lens has given me the best pictures from this camera (P-10). These of course drive up your cost, so learn the camera, then get the toys.