I have been going through the same thoughts as you, and im not convinced of the merits of the S5 for underwater use. It is great for wedding photographers who want jpegs that look great straight out of the camera, but i think as underwater folks we tend to be quite into our raw files and tweaking the colours etc.
Reports so far are that the S5 is not as sharp, and is slower to operate than the D200, add to that the extra cost i'm not sure its worth getting (and i do a bit of studio work, so it would make sense for me in a way).
I really want someone to buy one and try it out though, so, please, go ahead, get an S5!!

The fact the batteries are not cross compatible is a total pain too, i think for the rare instance you might want higher dynamic range it's probably not worth the hassle.
However, if you like to shoot in JPEG all the time then this camera will be great for you.
I'm still sitting on the fence, but im leaning towards nikon again