I have a D80 and like it, it is my backup to the D300, or used when I go on a shoot (not underwater) where I want quick access to two lens setups (like at the track or something with a tele and a wide). I'll use the D80 for a wide setup. I've never used it underwater. It is a good, solid camera in my opinion. Yes, D90 is more advanced, D300* further still, but like said above, if the D80 is in your price range, I don't think you would regret it.
At ISO 200 or 400 noise is not detrimental in my opinion. Above that, you will probably see some noise.
I think the D80 is 6fps max, I don't recall exactly, but if you're shooting with strobes you won't be shooting off 6 frames at a time anyway.
Focus is fast IMHO, and more dependent on the lens used.
Downsides, viewfinder mag is lower (0.75x I think), no live view on the LCD, smaller LCD etc.
I'm working on putting a 300s in the water now as an upgrade from my G10 because I found one from a previous owner I trust for a good deal and I'd like to have video capability, otherwise I wouldn't hesitate for a second to make my D80 the u/w camera.