Rule of Thirds explained
The rule of thirds can be very effective. Imagine a 'noughts and crosses' grid over the scene, then position your subject on or close to where two of the lines cross (some DSLRs offer a viewfinder grid option).
The human eye is more interested in images which are divided into thirds, with the subject falling along with one of the dividing lines, such as the necks of these swans.
If you can get a subject point (an in-focus eye for example) on the intersection of the horizontal and vertical grids (which this image clearly failed to do) so much the better.
Keep horizons level and place them above or below the middle of the frame (again using your rule of thirds lines) for a more dynamic image – the exception is when you're shooting a reflection, where a central horizon creates visual symmetry.
