Bang in the Middle

The temptation with most subjects is to position them in the middle but this is usually the worst place to put a them; although you can get away with it if the subject is symmetrical.

Subjects that fill the frame need only to be slightly offset to avoid the feeling of being stuck in the middle.

Balance

At all times you should aim to achieve balance - a quality hard to define, but which could probably be best described as a feeling of completeness. Simplicity is perhaps the key there.

To create balance you must find organization in the chaos, perhaps by lining up complementary elements. We find that shapes, repeated or mirrored, can be effective - abstract patterns too.

Patterns

Patterns reinforce the single motif, as seen with hard corals for instance, where the solitary coral polyp multiplied many times expresses the symmetry in nature.

Opposites Attract

Parallel alignment of subjects is another compositional trick that works. For instance, a near diver aligned with a far diver looks tidy and adds depth. Shoals of fish moving together are another obvious example. Unfortunately, it is your job to persuade your subjects to get lined up and, in the underwater environment, this is not always easy!