The strobe's guide number is for the flash exposure only and is mainly useful as a guide to compare strobes. This assumes the various manufactures use the same method to determine guide number. Underwater guide numbers are generally not the same as air as your surmise. However for underwater strobes the angle of coverage is more important than guide number - the wider the better since we tend to use the edges of the light from the flash.
Shutter speed will also affect the overall exposure with longer exposures increasing ambient light. For underwater photography I tend to increase ISO rather than decrease shutter speed to increase ambient light exposure since I want to stay close to the synch speed whenever possible.