The Tetra housing was first announced as supporting TTL flash. When it was released, that spec had been dropped. I've used TTL mode with my CP950, and was looking forward to the improvements, including exposure compensation, which are advertised on the C3030. What happened?
Basically, Olympus went cheap and proprietary when engineering the C3030 TTL flash system, and that precludes its use with existing underwater strobes. Read more for the excruciating technical details.
TTL flash uses a light sensor to determine the actual scene exposure based on the light from the flash. The nature of this sensor can vary. In the CP950 and CP990, a separate photocell next to the lens is used. This means that the TTL system can't tell how much the lens is zoomed, so if you're aimed tight on a dark object, the exposure might be based on lighter or nearer objects not in the viewfield. 35mm cameras (and my Fuji S-1 digital) use separate metering sensors installed in the optical path, which measure the actual light in various zones of the frame. In both cases, the sensors tell the flash to shut off when enough light has been received.
Olympus presumably wanted to avoid the disadvantages of the Coolpix separate sensor, but didn't want to add the expense of the additional sensors required by the traditional 35mm approach. So they decided to use the image CCD as the light sensor. Unfortunately, you can't measure the light using the CCD while you're taking the picture.
Instead, they cleverly engineered both camera and flash so that when you press the shutter, the flash fires a quick "test" pulse, which is then measured by the CCD. The internal computer calculates the exposure required for the real picture, and times the second, full power flash accordingly. As an engineer, I applaud the cleverness of this solution. As a consumer, I don't like it.
The system requires unconventional behavior in the flash unit. Olympus makes an external flash unit (the FL-40) that works, but it ain't waterproof. I suspect Light and Motion worked very hard to design an electronic adapter for the common underwater strobes, but I expect some (maybe all) existing strobes can't fire the two quickly spaced pulses with the precision required. Anyway, I was told that L&M couldn't get a solution working when the Tetra shipped.
Since I'm a skilled EE, I may try to work on the problem myself.
NOTE: As of this time, I haven't received confirmation from L&M about the accuracy of the above conjectures.