I gave this some thought a few years ago, and here is the product of that rare moment:
I was once a decent land photographer, in another era. As I looked into the techniques required for good marine photography, I realized that I had a lot to learn, and I didn't have all that much time to learn it. I then figured that with the amount of time I had to devote to learning the techniques I needed, by the time I got halfway decent, the camera I bought would be obsolete. It therefore made no sense for me to spend a bunch of cash on a good underwater camera.
I therefore bought a pretty good digital camera that would fit my point and shoot needs on land, and I got a housing for it. My logic is that my total underwater expense was simply the housing, since I would have bought the camera anyway.
With my basic skills, that basic camera fits the bill. I have taken many hundreds of shots, and some of them actually look decent. I was especially lucky taking pictures of the wrecks in Truk Lagoon, since they don't run away at the last second.
At some point, I will get serious. I will learn some more stuff, and then I will get a camera to fit my new skills.