Underwater Photography - what is the best angle for a single strobe attached to UW camera to get best pics?

None. It's all trial and error for the conditions you happen to be in and your subject. If it's back scatter that you're trying to eliminate, well, you aren't going to eliminate it all but you can get rid of most of it by extending your strobe arm out as far as it will go, away from the camera.
If it's shadows that are your concern, well, with a single strobe shooting against a solid back drop, you'll have them. You can't eliminate 'em with just a single light source. If your camera supports a second strobe, buy one. Having two is always worth it. If your camera doesn't allow a second strobe, you might be able to use a slave remote sensor with a hand held or mounted second strobe (if your housing mount supports a second strobe if not, hand held then). I believe Ikelite has a slave sensor that will do the trick. The sensor reads what the primary strobe is going to fire and will fire the second strobe at the same intensity.

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