I have done quite a bit of underwater photography over the years. Unfortunately not much during the last few due to other expenses in life. My family started underwater photography in the film days and have gone through many iterations....
Point and shoot (film)
Dedicated Underwater film P&S w/ strobe (Sea & Sea)
Digital P&S with small housing
Added slave strobe
Digital SLR (canon 20D) w/ Ikelite housing and DS50 strobes
Upgraded with macro port
Upgraded strobes to DS125 and upgraded to buoyancy arm sets
Upgraded to 8 in dome port
Maybe a housing for my 5Diii?
As you can see, there have been many trials and tribulations over the years of getting something that did the job but just didn't quite meet expectations. The film cameras were disasters and ridiculously expensive because of all the film The digital P&S just didn't get the image quality we wanted and the slave flashes were hokey at best. Finally with the Ikelite housing and the smaller strobes we had a good macro setup with a sigma 50 mm macro (easily the sharpest lens in my bag among lots of L glass). The DS125s allow us to graduate to more wide angle work. We still keep the digital P&S around for shooting movies.
I have given talks at local dive clubs and other places about underwater photography and I generally give people this advice. If you own a P&S and can pick up a housing for it or even get an entire used setup on ebay then do this first. DO NOT spend a ton of money on a setup like this. I know many people that enjoy diving but hate underwater photography because it "takes the fun out" of diving. If you like underwater photography, don't try to "trick out" a P&S setup...but if the quality bothers you jump straight to the DSLR.
If you know how to run a DSLR and are picky about using above water because of the quality differences then make the investment and get the underwater housing for it with some good strobes. I will stress GET GOOD STROBES that have TTL capability. The bigger the better as you will quickly find yourself selling small ones if you want to shoot wide angle! Run the camera in manual with 125/s and vary the aperture while letting the TTL do the work for you. If you are trying to take pictures of fish, it's impossible to change the exposure (or flash) settings as they swimming around.
Get a good macro lens and appropriate flat port. You will quickly find yourself covering 100 sq ft of a reef in an entire dive with a macro lens. If you want to do the over/under pictures like you mentioned, you are going to need an 8 inch dome port. A little tip for the over under shots is to rain-x your port so that the water sheets off on the above water side.
Here are some photos of the equipment we use now minus the 8 inch dome port...http://www.flickr.com/photos/braraigh/sets/72157619717802441/
And some of the results...nothing awe inspiring but good enough to cover an entire wall in my househttp://www.flickr.com/photos/braraigh/sets/72157619165504088/http://www.flickr.com/photos/braraigh/sets/72157618955549992/http://www.flickr.com/photos/braraigh/sets/72157621706718188/http://www.flickr.com/photos/braraigh/sets/72157608598935019/
I'm happy to give more tips and advice if you have specific questions.