After draining the pool with Neptune & the other sharks, you'll find him splashing around on the floor (remember to pick the assault shotgun up from the floor on your way over to him. Climb up the platform and you see a short cutscene where he thrashes around gasping for air.
Just push the large white box from the platform into the shallow water below and it'll create an electrical current through the water. Flip the switch, fry Neptune & the trophy is yours!