I've used neither, but the Leica looks to be an APS-C sensor, while the fujifilm has a 1/2.3 sensor. Given that sort of info, I can surmise that the Leica has better low light performance. The thing about underwater photography is that all that water does a wonderful job of reducing available light, and reducing the effectiveness of any flash. Your eyes do a wonderful job of adapting to the low light of a late winter afternoon just before sunset. But a camera just knows that there are quite few stops less light.
If ISO 3200 looks horrible on the Fujifilm, and you need Iso 3200, perhaps it'll be more usable on the more expensive camera.
The real test is between 4/3, aps-c or full frame camera in a dive housing, and this Leica.