I an not sure on the sensor size, but that is probably similar to the D7000. You have more megapixels than the D7000. But the D7000 is already pushing things with its sensor size with 16 mp. With the D7000, you lose resolution if you go over F8.
The NEX7 focuses through the view screen and not like the D7000 through a view finder. The camera will take some time to focus on something with the screen and that focus will probably not be really precise. I went from a Canon G11 to a D7000. With the G11, I was limited to the screen. That made my focusing slow and not very precise. On stationery objects or slow moving objects, I was OK. Small fast moving fish like Damsel Fish were virtually impossible.
The D7000 has a much faster focus. On the G11, I was doing well to focus on the outline of a fish. With the D7000, I can focus on the eyeball of the fish. Also one gets superior optics and a wider range of lenses with a DSLR.
If you are just going to do landscape shots, large fish and pretty much stationery macro. You should do fine with the NEX-7. Also the NEX-7 is probably considerably more compact a system than a DSLR. If you travel with a DSLR with your strobes, housing, lenses and ports along with your dive gear, you kind of become a sherpa in the airport.
It is really not a matter of which one is "better". It is a matter of which system will fit your needs better.
Since you are in FL, you might want to go down to Reef Photo in Ft Lauderdale. Reef Photo is one of about 3 major stores handling underwater photography equipment in the USA. They have pretty much all that kind of stuff in stock. That way you can see and handle the stuff before you make a decision.