I have shot with both cameras. As Larry C above has said, each camera has its advantages. The D300 is the top of the line camera for amatuers. It pretty much assumes that you know what your are doing and is set up like the old SLR film cameras. I also like its larger size and "feel". As already mentioned, the D300 has a pretty well made body and is reasonably bullet proof.
The D7000 is smaller and I don't like its fit in my hands as well as the D300. The D7000 also has those (to me) canned settings for people who want cameras that are a bit on the side of point and shoot. Now it has the full range of settings so you can ignore them. But I don't like having them on my camera and if I could, I would get rid of them.
That being said, the D7000 has noticeably better high ISO performance than the D300. Underwater being able to go higher on ISO can be a nice feature. If I am not pleased with my exposure on review, I can just fool with the ISO and adjust the exposure with the D7000. I really would be pretty limited doing that with the D300.
The D7000 also has a 16 MP sensor vs a 12 MP sensor for the D300. Now that is a nice feature but I would far prefer more dynamic range than more mega pixels. I have calculated that the D7000 is limited to an F stop of 8 before it starts losing resolution to diffraction. I have done tests with my 60 mm micro lens and confirmed this. So you can see, that the added mega pixels are not that big a deal because you only get the benefit at F stops of 8 or less.
We have all been waiting for the D400 for some time now and Nikon is taking their own time on this. I don't know if the D400 will be a simple upgrade of the D7000 with 16 MP and a similar iso performance or use the same sensor as the D5100 which is 24 MP. I don't pretend to know what features the D400 will eventually have. It could be a wait until we see one or it could be this fall. Who knows? I think Nikon has better security than the Pentagon.
Still, I would give the nod to the D7000 for underwater photography. But it is not a major difference. You can get superb shots with both camera bodies.
So what would I use? Well I would suggest that if you have a D300 & you have the housing, sit tight. I don't think the expense is worth the upgrade unless you have money burning a hole in your pocket (in which case have at it). Even with what I know now, I would be tempted to go for the D300 housing. I have discovered that you can get used housings for older Nikon DSLRs at a steep discount (check the classifieds in Wet Pixel). Heck, you can even get used housings for the D7000 at a nice discount.