Yikes, you better start snapping that shutter around your house to get some practice before you go. You especially need to be familiar with the housing controls.
I have a G10 and if your controls and features are similar here is what I use. Maybe you should think about shooting RAW so you have more room for error and extra room for post-processing forgiveness?
. Your on your on as far as the strobe settings go. IMO you should at least shoot your non-strobe shots in RAW so you can adjust the white balance in the Canon DPP software and not have "blue" photos shooting "Auto" WB in JPG mode. You can also use the "custom" WB setting in the JPG mode and carry a white card with you underwater to calibrate the WB at varying depths and light conditions. Shooting RAW will give you better control over your WB after the fact.
I shoot RAW and use the Canon DPP software to process the white balance and convert to JPG then Photoshop for the rest of the editing. I use the C1 and C2 features to save the strobe and non-strobe settings. It makes for easy changing between settings. Keep in mind the C1 and C2 features cannot save the ISO settings as they are controlled by a separate dial.
C1 for strobe shots: I shoot all my strobe shots in the "M" mode so as to have control over the F stops and shutter speeds. I change either of them as needed. Some articles on the web have stated that F5.6 is the "sweet spot" on the G10.
C1 Settings:
"M" Mode
F 5.6
1/250 shutter speed
ISO 80
"RAW" recording pixel mode
"Macro" focus mode
"Auto" white balance ("Flash" and "Day Light" are also suitable as the WB will be adjusted in Canon DPP)
Camera flash "on" (to fire external fiber optic strobe)
C2 for non-strobe shots: I use the "Tv" mode so as to set the shutter speed. When I press the shutter button half way to take a shot I can see the F stop in the monitor prior to shooting. If it's in red then it is not suitable for the conditions. I can slow the shutter speed and/or raise the ISO to suit the ambient light conditions. The higher the ISO the more digital noise will be in the photo so best to stay at 400 or lower. Shutter speeds slower than 1/80 will likely result in blurred moving objects.
C2 Settings:
"Tv" Mode
1/125 shutter speed
ISO 80 (higher depending on conditions)
"RAW" recording pixel mode
"Normal" focus mode
"Underwater" white balance ("Auto" and "Day Light" are also suitable as the WB will be adjusted in Canon DPP)
Camera flash "Off"
