Welcome to the club... Fun but can be frustrating at times. First of all I declare that I am still learning with my UW craft, although on land I an be considered old-hand, but not having dived too frequently I am still quite green in the underwater sense.
I dived in the past year with S60 and currently S70 (don't ask). And have recently purchased a manual Sunpak G-flash. Both blessing and curse.
Someone here (Kthan?) mentioned before - shot within the limits of your equipment. The on-board flash when used judiciously, i.e. near subjects, can give quite decent photos.
Lionfish, Maluku 05, S60 on-board flash
Once you are a little further from your subject, with on-board and small flashes, the strobe may become completely useless. Some say that you need big (BIG!) strobe and wide (WIDE!) lens.
Shutter speed and aperture would relate to the brightness of the scene. You can set the shutter speed and aperture such that ambient does not play a part, and only the strobe's lighting is seen, or a mix of both. This is an art.
Anemone Crab, Maluku 05, night dive S60 on-board flash aided by dive torch by my guide
My strategy of using the S60 was:
(i) shoot in program, since I had not much light to play with.
(ii) forced flash when needed, but depending on distance, might dial in a -0.5EV.
(iii) also dial in a -0.5EV to -1EV for ambient light
(iv) shot in raw, to get more control over PP (post processing)
More photos to encourage you to shoot without adding a strobe (I am assuming you do not understand enough about photography and light and need to understand more before putting precious dollars out - if I am wrong please tell me).
Frogfish, Manado 05, S60 onboard flash
Nudibranch, Manado 05, S60 onboard flash
So I got my Sunpak. Boy was it a pain to use. Completely manual. Try that during night dive - you need 3 hands, one to hold camera, one to hold dive torch, the third to adjust the power output. And being completedly manual means I need to understand lighting very well, and be able to adjust the output judiciously. So far, the common outcomes for close shots are first shot, out, either too dark or too bright, second shot usually ok. Medium distance shot, back to the drawing board. No hint for you, pal.
A shot to illustrate my point.
Black-spotted Puffer, Manado 06, S70, Raw, Manual 1/60 f8 Sunpak GN setting unrecorded
If you wish to go and grab a strobe anyway, stay away from completely manual ones. I know, I have 27 years of photography behind me, a sound understanding of light, GN, distance, etc, and I am frustrated when shooting with manual strobes. Mind you I started with manual flash guns in the 70s, with cameras that don't need batteries to work. Sunny f16 was the first rule I learned.
Or put it another way, Lovell's recommendation on the Inon flash is very sound. I am considering it, just that Home Affairs and Finance Ministry (read wife) not giving approval. And I also want to work on my fundamentals and understand how to work the flash I have within its limitations before splurging out more cash. That way I appreciate every little feature I get, and understand before I move to the next step.
Once you get around this then perhaps some of us can show you how to hook up the flash to the camera. Easy.
Just my 2 cents. And sorry for the rather long post.
Oh btw, C&C welcome (not meant to hijack you thread).