I agree that Fredo didn't know, but for different reasons.

If he did know, then you have to ask when did he know?

What conditions changed while he was out on the lake that would have suddenly made him know that didn't exist before he got into the boat in the first place?

And by getting into the boat in the first place, he was clearly demonstrating that he didn't know what was coming.

As dumb as he was, if he know that he was about to embark on a one-way fishing trip with Al, he would have made some excuse right on the spot and not gone.

And given the the fact that Connie was standing right there, it's doubtful that Neri would have killed him right on the spot.

By calling Anthony away at the last minute, it is clear that Michael's plan was for Neri to have Fredo make his final cast out in the middle of the lake with no witnesses.

So if Fredo would have begged off before leaving, the plan would have been put on hold.

But by getting into the boat, Fredo was is showing us that he thought he was simply going fishing and had no idea what was coming.