The latest micro 4/3 camera - the oly omd is outstanding. IQ right up near apc SLRs. Great lens selection from both panasonic and oly.
A very affordable housing from oly, and even a quite decently affordable one from nauticam, who have lowered their prices a bit.
The sony nex has serious issues of almost no lenses of good quality for underwater use, and none on the announced horizon.
The canon mirrorless camera only has 2 lenses, but an adapter for all the other canon lenses. The problem are 2. Canon has yet to make a contrast detect auto focus system, that is competitive with sony, panasonic, and olympus. Even their latest g1x is really not very good at auto focus lag. The other problem is sensor size. By choosing a full apc sized sensor, they are going to be required to have the same size lenses as the DSLRs, to get the same aperture and focal length.
For me, the micro 4/3 sized sensor is wonderful - allowing significantly smaller lenses, while still having killer IQ. And there are already many many lenses to choose from that are great for underwater - including macro, fish eye, fish eye zoom, and so on.
The panasonic camera bodies are not as good, in my opinion, as the olympus - as photographers tools. When using Aperture mode, with flash, they have no way to fine tune the slowest shutter speed, to prevent motion blur, of fast moving subjects like fishies. So if using A mode with strobe, the shutter speed will often be 1/60th which is too slow.
The panasonic cameras also forbid the use of auto iso in M mode. So really, you will be forced to shoot full Manual all the time, underwater.
The oly cameras have settings and adjustments for all that. They really are better photographers tools.