Hello 
This is my first post here on this forum so - Hi Everyone! 
I'm not the most experienced person here on this forum compared to a lot of other people (there's a lot of folk with more experience and skill here!) but I will try to let you know what works for me. I have never dived in Galapagos, but I have recently come back from the Maldives and the settings I talk about here were the good base line for shooting there.
I shoot with Nikon D60 in the Ikelite housing with the 8" dome for wide angle and a flat port for macro. I have used Nikon 12-24DX, Nikon 10.5 fisheye and Nikon 105mm VR micro lenses. I also have a single DS125 strobe which I leave in TTL.
I shoot in Manual mode and I have the ISO set to 100. First I would try to get the blue color of the water the way I like it. For this just shoot a couple of frames at the beginning of the dive into the blue, and change the shutter speed. Set the aperture to something that will give you a good depth of field (for wide angle, I would set it to f/5.6 or f/8 to start with). Then set the shutter speed. I would start off 1/125sec and if it's too dark I would go to 1/100, 1/80, 1/60 and so on. If its too bright, go 1/160sec or 1/200. If at some stage you'd need to go too slow (slower than 1/60), try to change the ISO up to 200 instead of going 1/30sec. This will allow you to avoid the motion blur.
For macro lens the above would depend whether I'm shooting something really small or really up close (in which case the f/22 or more and full power out of the flash is what you need) or something that's further out (like shooting clown fish from about 0.5m to 1m distance to get them in frame with a bit of anemone; in this case I would set f/8 or f/11 and again, blast them with flash).
You would then control your exposure with either the flash power (in TTL you might change the exposure adjustment or you can switch your strobe to manual and have it going at more or less power).
You can also try to change the shutter speed to control how much ambient light you have - remember that changing the shutter speed has no effect on the strobe, it only controls the ambient. Changing the strobe controls the exposure (but not the ambient), and changing the aperture controls the exposure for both the strobe and the ambient 
There's plenty articles here about the flash positioning and I was trying to apply the stuff that was written there, so there's no need for me to repeat that here.
Anyways - I hope the above is helpful and that I really didn't make it too complicated. By the way - there's a great site that I read about flash photography. It's not about underwater photography (all stuff they talk about is land photography), but it's all about off-camera flash. You can learn a lot about controlling your exposure and your lights from there. And it turns out, having everything in Manual is actually very easy!
check them out: http://www.strobist.com/
Best Regards and good luck in Galapagos,
Mike