Since you are using a strobe there are some basic rules of thumb to consider.
Set your f stop small, f 5.6 or higher, to increase your depth of field.
Use your shutter speed to control the background lighting. Faster shutter speeds will underexpose the background hue and brightness, generally creating a more pleasing appearence.
Use your strobe controls to adjust the exposure of your subject.
If you have a dive buddy with a digital camera, you can ask him/her to shoot a few images at different shutter speeds to get a pleasing background hue, then apply those settings to your camera. You will need to know their settings, but you can extropolate from them to determine your settings.
For instance, say they are using ISO 100; Shutter speed 1/60 sec; an aperture of f5.6 and this gives them an acceptable background color.
For your ISO 400 film, a shutter speed of 1/250 sec. at f5.6 or 1/125 sec. at f8.0 would create a similar exposure. Then the chief variable you will have to work out is adjusting the strobe for closer in subjects.
What you are trying to do is reduce the number of variables you have to deal with. With experience you will be able to make educated guesses and the exposure latitude of color print film will give you some wiggle room.
After you get this all worked out, then you will get a digital camera,,,,,and another,,,,,,,and another,,,,,and another.