When I started UW photography with just one strobe time ago, I spent many hours in my room shooting different repetitive scenarios and moving the single strobe to understand how the shadows are created. Also, modifying the amount of natural light from my window at the same time helped me out to get more information about lighting subjects. You don't have much time underwater to test all these things because of the limited free diving time, so better try it out out of the water. I would suggest you to put the strobe on the top but move it slightly to the opposite side of the subject to get a less plain pic and more shadows in the background.