Regarding Derway's remarks about the D-7000 I would agree that it is a great mid-range DSLR as is the Canon 7D but I see no evidence to support a large difference in auto focus speed. In fact the image-resource reviews give the NEX-7 over-all better AF speed scores and the DPreview has the cameras performance about the same with a higher overall score for the NEX-7. At image-resource the Nikon D-7000 was tested with a prime F/2.8 lens while the NEX-7 was tested with the 18-55 F/3.5-5.6 "KIT" zoom hardly apples to apples and the NEX-7 still did quite well.
I also agree that most of the photographers at this and many other sites could get along fine with 12MP and never see the difference gained by higher MP sensors. I am sure however that when given the choice between the coming Nikon D-800 with the Sony 36 MP sensor and the coming full-frame Sony with the new Sony 24 MP sensor many will argue that they need the extra MP's and won't be worried about having to buy an extra T-bite of storage.
The 24 MP sensor in the NEX-7 only seems to be out done by the NEX-5n when you get above ISO-1600 to 3200. I don't see many U/W shooters using those ISO levels much.
I have used both NEX-5 cameras and all the Olympus PEN cameras underwater. I have also used the NEX-7 out of the water and for me the NEX-7's upsides have nothing to do with the MP count. The thing that sets the NEX-7 apart and makes it the best mirrorless camera to date are the thing that you don't find on the other cameras. The OLED viewfinder is much larger than any other APS-C sensor camera on the market and is in fact just a hair under the size of a full frame camera. To date I know that Nauticam has released a back to allow use of a large 45 or 180 cameras on top of the already large camera finder and that Acquapazza has a like design in the CAD stage. The OLED is the best EVF to date and has a great deal more information in the display than does an optical viewfinder. The tradeoff of course is that it requires more power. The other big upside over the other mirrorless cameras and DSLR's as well is the Tri-Nav system which in manual gives one touch access to ISO, shutter speed and F/stop on three different dials. These dials can be set to any shooting combo to fit the shooting style. These two items alone make the NEX-7 a compelling choice against both mirrorless cameras and many DSLR's in the same price range.
While Canon, Nikon and Olympus/Panasonic M43 all have the more compelling lens range at this time it appears that many new NEX lenses will be coming sooner rather than later.
Phil Rudin