The DSC-P1/P5/P9 sereis are attractive for u/w use because of the availability of inexpensive Sony housings. I have a P5 with an Ikelite DS-50. Here are my observations.
All the P1, P5, P9 cameras have 1 major drawback: there is no manual exposure mode. Therefore having taken some 500 pictures with this setup, I would strongly recommend you get a strobe which itself is manually adjustable. I have the DS-50 which is a good strobe, but it is not manually adjustable. It uses a slave sensor that sees the camera flash and automatically adjusts the strobe output. So the strobe output is variable, but I can't control it. This works fine most of the time, but I have no way of changing the exposure if I don't like it, because both the camera and strobe can only be used on full auto. There is a +/- EV adjustment option on the camera, but it doesn't work when the flash is on, and the flash has to be on to fire the external strobe.
Therefore I recommend you buy either the DS-125 or YS-90, both of which are adjustable. You will need a sync cord with a slave sensor that looks at the camera flash. Both Ikelite and Sea and Sea sell these specific to their strobe. Ike sell a kit which includes the handle, arm , strobe and sensor together.
I also strongly recommend you tape over the camera flash, as it fires straight forward, and gives a great deal of backscatter from particles in the water. I use a piece of card taped over before I put the camera in the housing. There is still some light which escapes for the strobe sensor to see. Be careful to tape over only the flash and not the exposure sensor, which is a small round window next to the flash.
Also I have found the strobe works only for about 1.5-2 meters. I like to take wide angle shots, and they are always too green. On one trip I could compare my pictures with my buddy's pictures, who was using a red filter. His pictures were superb in terms of color balance. So I just bought the Sony red filter for this situation. I have not used it yet but I expect a major improvement.
Please take a look at my pictures at
http://homepage.mac.com/jetlife2/diving/
Thanks
Gareth Richards