Are you shooting in Raw mode with your G9? I use the same camera for diving and don't seem to have those issues but the environment is a little different. With manual WB settings you often end up maxing out to one end of the scale which can result in washed out color, you usually need to bump up saturation either on the camera or in post processing to get things right. Also if the tank is too blue no amount of WB adjustment will help, some of the best pictures I've taken are at warmer color temps around 10K or so.
The "graininess" is related to sensor sensitivity, your iso is either set too high or you've got camera issues. Unless you're trying to take a fast shoot to capture motion your iso should be set very low for your tank.
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