I had to start from scratch because all of my gear was stolen, and decided to switch to the D300. I just picked it up today, and after playing with it for a little bit, I realized how much I have to learn. The controls are nothing like my 20D. I'm sure that in the long run I will be happy. So far I have the 18-200mm Nikon lens and I just ordered the Tokina 10-17mm fisheye zoom, and the Nikon 105mm macro. I'm excited to have a camera again, but it is kinda frustrating not knowing how to use it. Well I guess I better get back to reading the manual.
Welcome to the Nikon family!
Here is where you can post questions. What's really cool with the D300 is that it's so adjustable. You can make your own menues, you can assign almost anything to any knob, you have two wheels to turn and sooo many options and settings.
I just got my D300 yesterday as well. I switched from D200.
You have picked very good uw-lenses, but note that the 18-200 is not a top notch land lens. It's an OK lens with a good range. You will get many great shots with it and the VR is really nice. But if you feel that the D300 is not meeting your expectation under some conditions (low light e.g.) with the 18-200, concider getting a more special purpose lens as well. The Nikon 12-24 is a very nice lens is you are into land based wide angle.
To get fantastic land based flash shots, you can get the small and cheap SB-400 and make your own a-better-bounce-card. The results are amazing (I have done that).
Oh, and finally, before you go on your expensive diving trip, make sure your system is honky. Check all lenses that you get perfect focus (no front/back-fokus) and that the D300 housing's AF is spot on. Mine is superb, the D200 was good too (even if it's AF wasn't half as good) but I've had several new lenses that needed (more or less) service/adjustment. The service at Nikon was really fast, for free and the lenses came back 100% sharp. The problem is that I didn't check this before my diving trip...
None of this is typical for Nikon, you get it with any brand, but it's worth mentioning over and over.
The D300 liveview has got alot of negative feedback. It sure sucks in many ways (and so does most other D-SLR liveview!), if you use the liveVew Tripod mode, Manual focus and zoom in the display, you have the most fantastic macro (actually any static subjects, not only macro!!!) manual focus system ever made. Partly due to the amazing display on the D300.
All in all the D300 is an amazing camera with an AF that really works spot on (even in low light!!!), but when it comes to low noise high ISO and improved auto WB, I dunno (well I've only tested the camera for a couple of hours...). So far I haven't felt _that_ much of a difference compared to my D200 (indoor WB is still better when set manually and there is more noise even in ISO 400 than I like). But, as I've sold the D200 I can't really make an A to B comparisment...
/Fota