Like you, more I look at the new SoundDevices recorder more it looks better suited for the A7S and the GH4 for underwater usage.No need record RAW, as the cameras cannot deliver it, 5inch screen will drive smaller housing and the SoundDevices recorder as only swtches and buttons for controls, no future issue here that we might have with the Atomos device if they change the Shogun firmware and messed up the screen layout for the Nautican housing buttons configuration.
I don't mean the Shogun is bad, I own one and it works very well, it just seems not to be the best option for underwater application.
About the BackMagic cameras, they looks nice and small, speedbosters likely to come if those two new cameras are becoming popular.