Wrybosome,
From you answers, I would suggest one of 2 canon cameras: the G11 or the S90. Both cameras are very advanced point and shoots. They are both under $500, but both are over your target. Canon does make a decent housing for the G11 for around $200. The G11 is about $450 and I think the S90 is about $350. You could add a strobe to it later. I believe that Canon also has housings for the S90. The S90 is pretty similar to the G11 and $100 less. So it would probably be your choice. The Canon housings are OK. I think that there are better ones out there but they cost quite a bit more. I get the impression that if you learn the proper care and feeding of housings, the canon housings are very dependable. If your housing handling is not that good, you can easily have a flood which means finis for the camera.
Both cameras have RAW. RAW allows you to correct white balance in post processing. I believe that having RAW is really important in an underwater system.
If you dive to any depth at all, you will want to have a strobe. There is no red light below about 15'. So if you want to see the "real" color spectrum, you have to take the light down with you. But in 30' or less, on a sunny day, you can get nice photos with ambient light.
Good luck