I am in the same boat, except I do want to use it on land as well.
So what are the underwater features and functions of any camera that matters? Image quaility. Raw mode. Killer auto focus. Easy to use M mode User Interface, (easy to adjust fstop and shutter speed without jumping through hoops). Easy to use and adjustable TTL and manual strobe controls, even in M mode. Easy to use A mode with flash, where you can set the slowest shutter speed it will use to 1/100 or 1/125. High flash synch speed, so you have more ability to isolate the subject, and more room to maneuver, before changing ISO.
The only 2 systems out right now are the micro4/3 by panasonic and olympus, and the sony nex 5n or 7.
The nikon 1 inch system is not good enough, nor enough lenses to consider. The new fuji system is way too new, and not enough lenses, and fuji traditionally has less than stellar auto focus systems. Could be great though, and oh those fuji colors!
For me, the sony nex system is still too limited in lens selection. And the nex 7 too highly priced, compared to what you get. The 5n has the perfect sensor, but annoyingly limited user interface, for using M mode, underwater. Folks use it and love it though, and the price is sure right. sony nex AF is not as good or quick as the best micro 4/3 cameras, either, and underwater, that is a crucial difference, for me.
The panasonic gh2 or gx1 have housings, or will have. There are lots of housing choices and lots of lenses including macro and wide angle. The do not have a usable A mode with flash, since the shutter speed will go down to 1/60, which will blur moving subjects too much, and you can't change it. The auto ISO is useless, as you can't tie it to shutter speed, or anything like that. They have "intelligent ISO", which decides for you, after you push the trigger, how high ISO to use, to stop blurring... You can't use M mode with auto ISO either. The panasonic cams do have the best auto focus.
The oly pens are much better photographer's tools. They all have a useful A mode where you can set the slowest shutter speed that will be use when the flash is on, and you can set in a slowest SS that will cause it it bump the ISO in auto ISO mode.
Many folks are using the epl2 and or other, more expensive housings. But like the nex5, the user interface is limited to a very few buttons, so you have to use menu buttons to adjust the fstop or shutter speed in M mode. This works well for them, after getting used to it. And it sure is cheap if you get it with an oly housing.
I tried one for a month, (not underwater), and sent it back. The very limited UI, and the lack of an EVF (without paying a lot more), and the complete impossibility to do any continuous shooting of a moving object, since you cannot see it to track or follow it on the LCD or EVF, they just show the shots you just took!
Now I am very excited, finally, by the new oly omd em5 camera, just coming out. It has 2 dials for adjusting SS and fstop, and the latest M43 sensor, and very very good auto focus, built in evf, M mode with auto ISO, and adjustable slowest shutter speed in A mode, with flash.
Oly is making a housing for this camera, which they never did for the other high end PENs, such as the ep1,2,or3. Which makes it more affordable than buying a fancy alu housing.
The camera is just coming to stores in the USA now, and the oly housing won't be ready until june. We don't have any real reviews out yet, but it sure looks like the killer cam for me! I've been hoping to get an EVIL camera for a long time, and tried a few, and this one is finally going to do enough that I want.
Good luck