Bokeh with wide open Aperture
Clownfish move around a lot, if you shoot a wide open Aperture (or a slow enough shutter) you can bring natural light into the exposure. For this try using "Spot" focus on the fishes eye, knowing that your depth of field will be very slight. The effect is similar to flash-fill on a wide angle scene, with a mix of artificial and natural light. If you are ever unsure as to if a flash was used, look for red, orange, or yellow- these colors are the first to filter out underwater, and generally indicate artificial light was used.