I know that Fredo is dumb. I also know that A) Fredo has a very strong instinct to run at the slightest hint of trouble, and B) Fredo was never established as the type of person who would ever go fishing with one of his brother's hit men. So, while I do think it's plausible that Fredo could be dumb enough to think that he's back in Michael's good graces, I don't think that it's plausible for Fredo to want to go fishing with Al Neri. It's out of character for him. Now, maybe there's some backstory in the book that explains how Fredo is such an ardent fisherman that he'll jump at the chance to go fishing with anyone, anytime, even if it's with his brother's hitman on a small boat. Or maybe Fredo and Al Neri have a long history of hanging out together? But without that kind of context, I just have a hard time believing that the character would do what he did. The scene felt "forced" to me, and the only thing that saved it from being cringeworthy was John Cazale's superb acting chops.