Hi Oliver,
Most of the time people are too shy to apply contrast to a photograph but in this instance I feel you have applied too much contrast. While it is important to maintain contrast in certain parts of the image, particularly to maintain life in the water, applying contrast globally can cause some of the darker areas of the picture to disappear into black.
For this reason it is often better to selectively apply contrast to an area as required; it is possible to do this in Aperture using the adjustment brush.
All I have done to improve the photograph below is use the Shadows/Highlights tool in Photoshop to quickly open up shadows. All I did then was crop a small amount off the top and the bottom of the picture and that is all you really need to do to improve this image.
Cheers,
Anthony
Image Doctor's edited version
