Wow, I think your question has rocked most of us back on our heels. What features of the D80 and 400D/XTi attracted you compared with other makes? No Live View, of course, on either camera but I imagine you ruled out the Olympus E-410 on other grounds.
Gordon's D80 Review
has a nice things to say about that camera and, when comparing it to the 400D, he remarks that it is "built to a higher standard". I am sure you have also read his Nikkor DX kit lens group test
which includes the Nikkor DX 18-70mm you mentioned and has some nice sample landscape shots. I presume a fairly fast lens would be the ideal solution for the underwater work but looking at the price of underwater housings they put a serious dent in any budget. Similarly, I imagine low camera sensor noise at fairly high ISO is also desirable.
Don't get too concerned about your choice of camera becoming obsolete. True, both Canon and Nikon tend to refresh their consumer models every 18 months or so but that doesn't mean that the earlier models suddenly stop taking great photographs. There are many owners of EOS 20D cameras out there who are still content not to upgrade despite the fact that it is now superseded by both the 30D and the 40D.