Backscatter is caused by particles that are suspended in the water which are illuminated by a strobe. They show up as unwanted white particles in your digital underwater photos. Sometimes the particles appear as large specks or large white blobs and they will take on the shape of the lens aperture and this will depend on how focused the particles are.
How to fix backscatter?
The angle at which the strobe is held can make a significant difference in minimizing backscatter. It is far better to move the strobe away from the lens, typically at a 45-degree angle above and to the side of the subject. Remember the effects of refraction under water, and aim your strobe slightly beyond your visual perception of the subject’s location.