Hi!
I guess that you should stick to Inon (used), Sea&Sea or Ikelite at the moment. Optical connection is probably best with the Inon, though you need a strong stomach and a stiff drink to make it through the manual with sanity intact.
It all depends on money: if you are going to stay with underwater shooting you will upgrade, so a more expensive and flexible strobe could be a better buy in the long run. If you have a wide range of power settings, then TTL is not so important; in any case, it is unreliable except in macro photography.
If you are having trouble with strobe illumination, remember that the strobe needs to be pretty close to the subject, but also at an angle to it. It can be surprisingly difficult to point a strobe in the right direction. Images with background illumination and balanced strobe light are often the most attractive ones. Shooting at night will need you to use a focus light mounted on the camera, and I have always found night photography a bit disappointing.
Tim
