Are you off your head? The d7100 is miles ahead of the d90 in everyway. Id say get the d7100 and keep you lens setup.
This is why I said "in my opinion", you may think its an upgrade, I do not, it's a simple case of differing opinions, but as my opinion seems to upset you in some way, I'll explain why its
my opinion.
I compare the D90 to the D7000 / D7100 as all of them have two control wheels for slightly easier manual control whereas the D5100 and D3100 etc. lost one so to me is more directed towards users who use auto mode or any of the semi auto modes. Other than updates (big and small) to the hardware and software inside, what big upgrade do any of them bring over the others?
Yes, its got more MP and a better AF system, better than the D7000...which in turn had more MP and a better AF system than the D90...which incidentally was phased out when the D7000 was released...because it was updated in the form of the D7000...which recently was updated in the form of the D7100...
I have a D7000, the D7100 to me is not an upgrade, it is an update that will give me some more MPs and better AF. If I wanted to upgrade that body, I would be looking along the lines of a D300 or an FX body.
Of course, the OP is free to spend his money however way he likes. If it were me, I'd possibly look towards FX, for most of your example photos you don't need hugely snappy AF except perhaps for birds, but you may appreciate a shallower DoF, a wider field of view and better low light capabilities in particular for your marine life photos.
That said, a D7100 and your current lens set up will serve you well if you want to go that route - but I'm unsure exactly how advantageous the D7100 will be over your D90 when looking at your photos. Double the resolution will be useful for crops but the D600 has 24mp as well and if you went for a D700 I believe it has around the same resolution as you already have in the D90.
I have a D700 and D7000 and I have to admit, the D7000 rarely gets used now, predominantly just for situations where the extra reach of a DX sensor is needed, even though it has more MP and I *think* it has snappier AF. You can google all the arguments about DX vs FX, but when I use the D700 it just feels better, images are clearer further up in the ISO range, viewfinder is brighter etc. It just feels nicer, hence why when I pick up a camera, my hand always drifts towards the D700 first.
