You won't need over 10 meg, I still have 2 - Nikon D200's which are 10.2 meg cameras, that are from 2006, I have no choice but to keep them, I have 2 underwater housings for them. I also have a D700 12.2 meg, and just ordered the D800 36.3 meg. With the newer technology, you get faster focusing, lower light capabilities, faster processing times. If you are shooting underwater, make sure you get external flashes, it will reduce the scatter in your photos.
This is what scatter is:
http://www.uwphotographyguide.com/ba...ter-underwater