I had the chance to do some qualitative, but at least, back to back AF tests with a full production (as opposed to a Nikon demonstrator) D7100 at the weekend, at the London Dive Show. Nauticam had brought one to show off their working prototype housing for the D7100 (which included their new vacuum pump system).
D7100 Housing is almost identical to the D600 I just reviewed - just tiny changes to positions of controls
Inner workings of the multi-selector in the Nauticam.
D7100 AF performance seemed equal to the Nikon D800 with 60mm AFS I was using. This is a big jump on the D7000. (Although the most noticeable thing was how much brighter the viewfinder was in the D7100 compared with the D800.)
The D7100 is therefore better than the D300 in AF, but not hugely. It obviously has twice as many pixels and yet, I'd expect it to be much better at higher ISO. Nikon has made such gains in this area since the D300. I'd also expect considerably better dynamic range, but haven't tested it. Plus smaller, but less robust, less RAWs in buffer. Larger screen, video etc.
But if you shoot UW in the tropics, are mostly interested in macro and don't want to make large prints - you'd probably struggle to see that much difference from the D300 in the final results.
Also we won't have to wait long for D7100 housings since the camera is physically so similar to the D600. Subal had a very compact prototype D600 on display and you can be certain that the D7100 housing will be almost identical - which should speed deliveries of all brands.
Subal D600 prototype (photographed here without handles) - expect everyones' D600 and D7100 housings to be almost identical.
In fact, at the show we were joking that housing manufacturer's need do little more than cross out D600 and write on D7100 in crayon.