Not really meaning to be snarky, but the high regard for the G9 has to be getting a little moldy by now, don't you think? After all, the G9 is more than ten years old, and sensors have improved quite a bit since then. I have one myself, but I retired it quite a few years ago due to the newer generations of sensors. One of the G9's recent descendants would probably be a much better choice for the OP.
For me, the ability to shoot raw format would be first, second and third priority. Raw shooting eliminates any need for custom WB, since the camera white balances by discarding sensor info. The raw file contains all the info that the sensor captured, so WB'ing in post carries no IQ penalty. As a bonus, the raw file also allows for a lot more shadow/highlight recovery without significant IQ loss.
Of course, that requires that the user is comfortable with PP'ing on their computer in the first place...
I think the issue is cost and performance. If someone just wants to buy an 'affordable' camera to take in the water that will perform well, i.e. compacts. Then Cannon (among a few others) have over a long period, had a range of good cameras.
The other issue, is most divers with cameras don't really consider themselves photographers. The don't always understand, appature, speed, ISO etc. They use a camera in automatic, point and shoot.
Not having white balance, means you need a strobe - a significant cost increase. Or LR/PS etc, which will significantly improve your pictures - if you can be bothered with post processing.
I still use a S95, granted I have strobes, lenses, and LR. But, truthfully the quality of the pictures are not limited by the camera, but my competence.
If I buy a fancy SLR and housing, my pictures are probably not going to improve that much.
White balance also means you can do good quality video reasonably cheaply, because (on the canon), the white balance also works for the video.
I don't pretend to be good or even that well educated with regards photography. But as some one was discussing with me recently - the magic figure is £500. If you can price the product under £500, it will probably sell well. The perceived cost is acceptable. (Note the GoPro also falls into this magic bracket).
Now, if the question is, I have 2-3 grand to spend on a camera, then you are not looking at a compact - and I am well out of my depth.
I am currently struggling to justify upgrading the S95's I have - a faster camera would be nice, but will my picture taking improve. LR certainly improved the quality of my pictures, but that is in the post processing, not the fundimentals.
(One up side of the S95, is the cameras are/where relatively cheap, so I have spares and am not terrified of a flood
)
Gareth
PS I also intended to say that the Canon would shoot RAW, for those interested.
