Composition - the Rule of Thirds
The Rule of Thirds is a compositional rule of thumb in general photography. The rule states that an image should be imagined as divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections.
In other words, you should make sure to place your subject in one of those imaginary intersections, rather than positioning it right at the center or too much towards the corners of the image.
Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject would.
Note that many judges of underwater photo competitions make use of this rule as part of the criteria of a successful image.