No need to do the treatment of the flash. Sorry Chad, I usually not. (maybe it is just my way, your method is good though… I usually use them when I use super wide angle lens… and see more dramatic effect…)
Pat. you might want to take a look of Zach’s brown series/ Morsie’s saltwater series. They have fill flash and they do not over expose the fish…
The reson you feel you will over expose the chromer is you already over expose a bit (because of the dark background… maybe)… I would tune the camera to normal exposure, and fill in the flash about 1/3 or 1/2 stop. and that sould be turns out right…
I do think set the flash light -1/3 or -2/3 stop would work just fine to reduce the fisherman’s shadow…
A good way of using a flash… use them lightly!