uriho,
when you have the 10Bar housuing already you have 2 solutions:
1. use a electrial cord and learn how to shoot in manual
2. use a optical tiggered slave strobe and shoot in s-ttl (Inon) or ds-ttl (S&S)
The G11 work perfectly with a INON D-2000 strobe in s-ttl mode giving you the possibility
to finetune the strobe output in 12 steps.
I recommend strongly to get a optical triggered strobe and not a electrical triggered,
especially as it will not provide ttl.
How important is ttl, well, first it depends on your photographic skills,
your knowledge about exposure and how you obtain a perfect exposed picture.
If you are handy with that you will have better results in all situations shooting manual rather than ttl because you
deceide the exposure.
TTL exposure regulates exposure for you based on parameters stored in the cameras computer.
This works mostly well on land, but under water and especially on WA photography it fails often as the variation in contrast and/or reflections (from silver fishes for example) are very high and this often fools the camera, resulting in burned out (overexposured) parts in the pictures.
TTl works well on macro where one object is filling the picture and so ttl is able to measure the right values for exposure.
Reef Photo has a article about the various modes Inon strobes offer and the basics are similar for S&S strobes as well.
Reef Photo & Video Support Center - KnowledgeBase - Selecting the proper settings for INON strobes
Chris