I don't think I'd call it a problem so much as flash from the cam tends to flatten objects in images. Light from above is what gives fish and other underwater objects more of a 3D image.
Something I might try with your current set up is to shoot with a shutter speed somewhere in the 1/60 to 1/125 range with an aperture range of between F11-F22. An image taken in the higher end of that aperture range should have more depth to the image. That in itself could make the objects pop a little more.
Shooting at 1/15 really makes your images vulnerable to slight tremors, even while sitting on a tripod.