Hi
Will the new D90 be able to make better pictures than the D80 , and why ??? 
The answer to this depends on many other factors.
Theoretically, the slightly higher resolution can lead to sharper images, the capture of more detail.
BUT, only if your lens + port + shooting technique are sharp enough to deliver that tiny bit of extra resolution. If they're not, forget the D90.
As an avid D80 shooter i see nothing too exciting in the bag, neither did i expect it. The D80 is a great camera already.
Let's see...is there anything interesting for the underwater shooter, making a D80 to D90 upgrade worthwhile?
1) 12mp vs 10mp, possibly sharper results, IF lens, port, shooting technique are up to it.
2) higher density of the photosites means more diffraction issues at small apertures (f13 upwards maybe) - needs to be tested
3) 3 inch lcd vs 2.5 inch...do you need 3inch to check your histograms and sharpness? Of course not! You still need to zoom in for a sharpness check.
4) video mode....nice gimmick, but not needed, and you will certainly not be able to deliver high quality uw videos without proper light, and focusing/modeling lichts from our strobes are NOT enough, as pointed out in an earlier post
5) taking a video frame as a still: makes no sense to me, the video is a crappy jpeg in 1280x720 resolution, probably without the possibility of proper exposure control.
6) base iso of 200: certainly a pain in the ass for sunbursts, we need to close the aperture one stop, the flash(es) have to deliver double the light. of course you can use the iso100 mode, but the sensor is delivering best performance at iso200, so there will be some quality loss here.
7) 12bit NEF unchanged....14bit would have been a welcome addition here
8) noise: from all i've seen the d300 is no improvement over the d80/d200. just that the in-cam noise reduction is blurring the image a bit more aggressively.
9) ergonomy: nothing changed here worth mentioning, the d80 is already superb in this aspect. I know because i shoot exclusively while freediving, so size, weight, image quality and ergonomy are THE reasons i bought the d80.
10) live view: not at all important for serious uw use. the focusing speed is slow, only by using the viewfinder will you fully exploit the abilities of the cam's AF system.
11) AF system: no real change here, 11AF points unchanged, the d80 already has a fantastic AF system; it's tracking moving objects across the frame without any problems (i've had great fun with this while shooting air shows).
Here's what i expected for the upgrade:
1)12mp CMOS, of course the d300 sensor jammed into a smaller body
2)14bit NEF... this would prolly be a more noticeable increase in image quality, specially when looking at those smoooth gradients of background water, and sunbursts.
3)more speed! something like 6-7shots/second....the 3.5 to 4.5 is a very modest increase
4)more buffer! i pack 7NEF-bursts into the buffer (using a sandisk extremeIII), no change here
5)Live view...i don't need it, but people expect it so what the hell
6) MOST important: fix the screwy matrix metering of the d80. It's overexposing, it's unpredictable, it's unable to deal with fisheyes. I use center-weighted average and spot uw, both work and are very predictable. Right now we have no idea whether the metering issue has been fixed in the D90. I have my doubts though...
7) regarding ergonomy: a faster way to set a custom wb. like assigning a button or a combination of 2 buttons to this function.
All in all, the D90 is a very attractive cam, for people who are still shooting compacts, or older dslr's in the 6-8mp range.
As a freediver I'm really glad to see that NIKON is staying commited to building highly capable cameras using state-of-the art sensors from their higher-end lineup, but in smaller, yet ergonomic bodies. The D80 is a dream to use, I love it, and it's great to see the family growing!
I don't think the D90 or even the D300 are worth the upgrade from a D80 though. The next logical step is the D700/D3/D3x. Only makes sense if you shoot a lot, and anyway the best upgrade you can ever get is better composition and shooting style, which of course cannot be bought in a shop.
So...if you have a D80, don't bother with the D90, get wet and really work your 80!