Another big concern is packing for dive trip, with all the port, lens, and gear for interchangable system, it is going to be a nightmare. January is still a long way to go, hopefully the LX100 will be selling like a hot cake and housing manufacturer would take the opportunity and seriously thinking about making a housing for it. Even Inon should start consider making a new version of WA wet lens as their UWL-H100 lens are getting less and less compatible with the newer generation of point and shoot.
The camera is reported to be 26mm at 4K, this is indeed quite surprising as I would have thought more around 28mm. We need to see the physical size of the lens and how the zoom works before making statements but the UWL-H100 would work as it does on the RX100 Mark III at 24mm without vignetting
There is however a lot of fringing
One thing you certainly want to consider is that due to the multi aspect of the sensor the 26mm correspond in fact to a 24mm with crop on top and that means in terms of horizontal field of view you get the same of a 3:2 at 24mm which I think is around 73-75 degrees. This starts to be acceptable with a wet mate port and that will definitely work regardless of the shape of the lens
So best case Inon UWL-H100 with 104.5 degrees diagonal, worse case wet mate with 79.52 both are better than the GH4 with bare port or wet mate after 4K crop
You can actually use a wet lens designed for 35mm lenses over the 35 port but I am not sure about quality still this will be more expensive than the GH4
I am pretty confident a housing will be produced but am waiting for detail testing of the LX100 and the release of the user manual that really is what defines if the camera is going to be good