Long-lost scene in the script:

Neri: "I couldn't believe it, Connie. As I told Mike, Fredo was in the midst of a Hail Mary, his secret to catching fish, and all of a sudden he caught the Holy Mother of all fish -- it must've been a hundred pounds -- and it literally pulled him into the water! I tried to save him, but I can't swim, you know that. He must've drowned right there! By the time I got back to shore to call the authorities, and went back, I couldn't remember exactly where we were within a mile ...or ten... radius! I'm surprised they didn't decide to dredge for his body. But had they, I just recalled that the fish retaliated by shooting him in the back of the head!"