I have the Oly SP350 with a hardwire TTL connected to the housed FL20 flash. I can use the same rig topside with no housings. The hardwire TTL has been great as a beginner. The Through the Lens metering detects how much flash is needed and adjusts the flash output accordingly. The TTL of the rig works great underwater and takes great portrait pictures. Here's the problems I've noticed.
1. The housed FL20 fires out a very narrow beam. I've worked around this by using both the internal and external diffusers. The external diffuser is a pain to keep on the housing of the FL20
2. The TTL doesn't tend to work real well in super macro mode. To compensate, I can set the flash to manual and choose the output through the camera menu.
3. Firing another optical strobe has presented a few challenges. First the housing on the FL20 isn't conducive to having anything attached to it. So I had to rig up a way to attach a fiber optic cable to the outside diffuser. This new "rig" worked great for about 4 dives. I now have to figure something else out.
4. Without the cable the optically fired flash (Sunpak G-flash) tends to fire at any moment it catches sunlight and not fire reliably with the FL20 flash.
As a beginner with a little over a year of experience I can tell you that my biggest frustration has been my second optical flash. So much so that I'm debating dumping the FL20/Sunpak setup in favor of the Ikelite or Inon. I'm leaning heavily toward Inon right now. I've read somewhere that Ikelite and Inon now support the Olympus Hardwired TTL but I need to verify this in the coming year. I'm hoping its only bulkhead adapters or modified bulkheads because I really like the simplicity of the Oly housing for the SP350. I know that the Inon can connect to the housing of the SP350 on the internal flash port. Then through electronics built into the Inon flash, it suppresses your internal flash from firing, while using its own built in TTL-S to deliver the correct amount of light. Suppressing, or not using, your internal flash in most point and shoot cameras, including the SP350, is vital to things like battery life and recycle time (time it takes for the camera to be ready to take another picture).
Thanks,
Gary