This august I was in Red Sea and I did an early morning dive in the bay near the resort.
With my Canon G9 digital camera and flash YS27DX I had a lot of troubles because of low shutter speed. So I tried to use high ISO values (more than 400 ISO) but I noticed that noise was really a lot!
So I had the idea: fix 200 ISO and start to underexpose (1 or 2 stop) and compense the low light with more flash power! In this way I had higher shutter speed and darker background with the result of a more tridimensionality in pictures.
I hope this tip can help some underwater digital photographer!