It is not just a matter of filling the frame, light aimed directly at the subject means backscatter. If you dive in crystal clear waters this is not an issue the moment you have a bit of silt then is a mess.
The only case where you can use a single light is macro where you are so close that you can aim the light at 45 degrees and avoid backscatter
The only case where you can use a single light is macro where you are so close that you can aim the light at 45 degrees and avoid backscatter