It looks to me like your strobe is not aimed correctly (too far to the left...). Practice on land to see where the angle of light for your strobe is and then try to get your subject "painted" with it. That strobe is not particularly powerful and you are going to have to get close. To reduce backscatter you need to light with the edge of your strobe's output (easier said than done, I know!) and reduce the amount of water between you and the subject - get close. Practice moving your strobe head a bit, too.
Your subject doesn't look to be in focus really, either.
In getting close, remember your minimum focus distance of the camera/lens you are using or even if the lighting is right, the subject will be out of focus.
With your internal flash covered, you won't get any light (or at least enough to matter) if you don't use your external strobe. Can you adjust settings on that model - I don't see any controls from the web page??
I know they are trying to sell cameras, but that photo on their website is an unreasonable expectation, I think.
HTH