I use a G10 with a single Sea & Sea YS-27DX strobe. Here is my two cents worth:
1. I am a recent convert to shooting in RAW mode after years of shooting JPG. It is a must to get the best results on non-strobe shots.
2. I use the Canon DPP software that came with the camera to process the RAW files as respects the white balance. You can do it in PS if you have the plugin. I prefer the Canon DPP. Just a personal preference. I do the rest of my adjusting in PS.
3. For strobe shots I shoot in the "M" setting, F5.6, 1/250 to 1/500 range, ISO 80, "Auto" or "Flash" white balance setting which doesn't make much difference as I adjust the WB in DPP post-processing from shooting RAW. Shooting mostly in the "macro" setting. Some articles state F5.6 is the "sweet spot" on the G10.
4. For non-strobe shots I use the "Av" setting starting at F4.0 in clear shallower water adjusting to wider apertures depending on available light and depth. Non-macro setting. ISO 80 to start then on up to ISO 100 and 200, again depending on available light and depth. I use the "underwater" WB setting as it makes little difference since I will adjust the WB in DPP. Shooting RAW eliminates the need to calibrate your WB off a white card. The "underwater" WB setting can't compare to shooting in RAW then adjusting the WB in post-processing. Be careful not to allow shutter speeds slower than 1/80 or moving subjects will likely blur. Just up the ISO if you are already at F2.8 and can't get a decent shutter speed. I stay at ISO 200 or below to avoid digital noise.
5. I use the Custom 1 setting to save my strobe settings and the C2 for my non-strobe settings. You have to remember to adjust the ISO with the dial as the C1 and C2 settings cannot save the ISO settings due to the separate dial. Using the C1 and C2 settings make it easy to switch back and forth between strobe and non-strobe shots. For one it will save you from forgetting to re-set the macro and non-macro setting as well as the "M" to "Av" modes and turning on and off the internal strobe if you are using a fiber optic cable to activate your slave strobe.
Yes, the RAW files are large. I am more selective in what I save. If you still want to shoot in the JPG setting the above info still applies but use a white card to calibrate your WB on the non-strobe shots. If you compare the difference between the RAW and JPG results I think you will be sold on the RAW as I was.
Good luck with your G10. I think it is a great camera.
