It really helps to shoot RAW. Less advanced cameras shoot JPEGS. The camera takes the file, interprets it and makes a guess as to what it SHOULD look like. That often works well in nice sun lit situations. But underwater is funky. A RAW file records all the data a camera sees and you can use software to get the most out of the image.
First off, the spectra of light are absorbed by the water. You will not get any reds at deeper than 15’. By 60’, you only have blues and greens. I once took a photo of a queen angel at 80’. There was a grey wall of the wall behind it. When I looked at the photo later, I was struck that the grey wall was revealed by the strobes to be a riot of color of encrusting sponges (oranges, reds, and yellows), In order to get the colors, you have to bring a full spectrum light source down with you - either a flood light or a strobe. And your lights have limited range so most people try to get within 5’ of their subject (or better yet closer).
Now you can get good results with advanced point and shoot cameras. There was a guy here who got great shots with a Canon G8 and small strobe. He really knew how to use it.
For posting on the internet, you do not need a very big file size. A point and shoot will work. For big enlargements, you need about 24 megapixels.
The thing is a camera, strobe, and housing are not cheap. Each housing has to be made specifically for that camera so it is a low production which increases the cost.
Another thing is underwater photography is challenging. The water is very challenging envionment.
Also, photography is distracting. So you dive skills need to be solid lest you get lost in the shooting and put yourself in danger (or you collide with the reef and damage stuff).
If this information does not deter you, you can check out the camera gear choices at the Backscatter web site. They have a bunch of information.