[Skindiver]

I think the laser sight idea is a cool one if it would work. Its main problem would be tracking moving fish .. as you need to 'lead' or aim ahead of the fish and your reliance on the visual dot would then be a hinderance. But even better is the suggestion of a torch. I have recently ditched wearing sockies in an effort to reduce the amount of kit and kitting up i find myself doing lately so having a separate torch for under ledges and in caves would be a definate no no.

I have freedived with competition divers in deep dirty water who have tied a small torch to their mask straps which comes on with increasing water pressure and goes off again with decreasing water pressure. No fancy pressure switches though. They unscrew the torch head until the globe contact only just stops making contact with the battery. At some depth the water pressure pushes the torch face inwards against the O ring and contact is restored and the light comes on. Considering the time spent under water a few batteries should last easily the day.
No switches and auto on and off. A simple bracket and fix to the underside front of the gun and it illuminates and possibly shoots where you shine. Alternatively leave it on the mask strap and it shines where you look.

This must work.
Skin.