I tried to find pressure/depth rating for that gizmo, but could not find it. I would be surprised if it is rated more than 10'/3m. This may be OK for shallow snorkeling, but nothing else. Light loss at those depths is trivial.
For SCUBA underwater, you want at the very least 100'/30m, better 200'/60m. Those housings are MUCH more sturdy. Not up on it anymore, but had Hugystrobe back in the day for OM4 and Pentax LX in housing. I also agree that you need MUCH more power underwater, more on par with a studio strobe than a Canon flash. Many UW strobes also have modeling lights built-in, very useful for macro at depth.
I used TTL flash no problems back then. Just do the exposure compensation as needed. Having access to controls of the flash underwater is certainly advantageous.