Backscatter! All underwater photographers have experienced it. All of us hate it. It's ruined many a picture that would have been fantastic without it!
Here's a technique to help prevent it.
In this diagram, you can see that with the strobes aimed at the subject, the illumination from the strobes intersects at some point between the camera and the subject.
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The result is that any particulate in that path from the lens to the subject is illuminated, producing what is known as backscatter.
The technique is to actually aim the strobes outward, so that just the edge of the illumination catches the subject. Uses the 'Rule of Thumbs' as a guide. Rest your hand, (relaxed) above the camera with your thumb pointed along the axis of the lens, like so:
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Then point your strobes along the axis of your fingers. Granted, this will vary as different strobes have different angles of coverage, but just use it as a general guideline.
Using this concept, even if there is particulate matter between your lens and your subject, it is not illuminated...
