I also have a G9 but not used in much in a while. If I recall correctly, you do not get flash TTL when in Manual setting, leading to your issue. So you might try Av, Tv or even P settings to get TTL flash. If you use Aperture priority, you can set the f-stop to perhaps f8 and have a better chance for good exposure.
If you want to stick with manual, you could dial down the f-stop and dial up the shutter speed, review the photo on the spot, change the f-stop and try again. I had two main frustrations with the G9, the lack of TTL in Manual and the shutter lag. An open f-stop and close photo's will be difficult not to over expose.
this is a great piece of advise and it just shows how much i understand about this camera. i also wonder why i never bother trying switching to other modes.
can i also check with you what's the normal exposure you use underwater? due to the flash being so bright, i always ensure i'm shooting photos underexpose. depending on the depth and object size, i'm always between the range of -1 to -2 so that the photo can be compensated by the flash (as in hoping the flash will not be too bright like the above photo).
about the f-stop, i tried before jacking it up to f8 or even f10 but that would result to a slower shutter and a higher ISO. Knowing G9 will be noisy at ISO400 and the response of the camera will be slower, I always end up having OOF photos. I also noticed the higher ISO, the more backscatter there is.