Common Merg shot today with a shoulder stock mounted Nikon D200, 80-400mmVR at 400mm, f8, 1/320th sec. ISO 400, auto exp mode +.67, fill flash SB 800 -2.0. BG is the reflection of a south facing cliff and the sky. PP'ed in CS3, cropped about 40%. C&C welcome.
Nice find and I agree the water colors are quite vibrant. :) I wish the duck would swim toward you and the angle were lower. You could tone down the whites some more and maybe crop a bit tighter.
I agree with Axel's suggestions. Also, the positive EC probably hurt you here. There's a lot of dark surrounding the white areas on the bird. The result of metering darks is that the camera will let more light in. Negative EC was probably called for here.