That's a nice sharp shot with good use of DOF to bring the subject out against the background. Given it was your first dive with the camera, you have good reason to be happy! Two suggestions:
First, depending on the relative positions of subject and background, using only one light, placing the light off to one side might throw a little shadow behind the subject and improve isolation. In this shot could you have placed the light lower and to the left or right, using the seahorse's shadow to shade some of the background?
Second, I don't like the subject's orientation, even if that is what they actually do most of the time. Since it's hard to ask a seahorse to sit up nicely, and you can wait forever for them to think of doing so on their own, maybe backing off a little to leave room to rotate and crop the image slightly would make for a more eye-pleasing pose. I am trying to make myself back off more often for just this reason
Edited by troporobo, 21 March 2017 - 10:28 PM.