For positioning etc see
http://www.yorkshire-divers.co.uk/fo...ad.php?t=14425
The range of guide numbers will be the range of light settings (effectively min to max). The only way to work out which is best is to play with the kit underwater and take some pics. My advice would be to start with a set of shutter speed/exposure that can be used for most things (example F5.6, 1/100s).
Even easier would be to set the camera to auto and take a pic of something with the strobe set to half power, check the pic, increase/decrease power on strobe, retake etc until you have some rough idea on how the strobe performs.