That looks like a happy duck. I like the BG grayish/green color and the water drops on the head. I personally don't mind the mid body crop but I think I would like to see a little water between him and the bottom of the frame. Could maybe use a touch more sharpening?
Personal preference, I think the amount on the left is fine. Gives him something to swim into. If anything, maybe a tiny bit off the top for more of a pano look but again, that's a personal preference.
The water droplets are a nice bonus in this image Eric and I like the open beak. I would crop a little off the top and left to bring the composition more into balance and a tad more USM. Also if you had pointed the camera down a little more you would have more water below the duck. Many thanks for sharing and look forward to your future posts.
When it comes to portraits, or close-ups, you don't need as much room in from of the subject for it to swim into. You could crop more than half of the empty space and still get away with it. See my recent post of Mallard for an example of what I mean: http://www.birdphotographers.net/forums/showthread.php?t=25779 . I agree with Judy about pointing your lens down lower though...and getting down lower yourself if at all possible at that location (e.g. lying flat on your belly). Face could use a bit more USM, but excellent dynamic expression on the duck!!