Actually, Fredo said hello to Al at mama's wake and Al somewhat awkwardly returned the gesture.

They may not have been the best of friends, but certainly were not enemies. Al was a trusted member of the 'Family'. If he was asked to accompany a Corleone on a fishing trip, then that's what he did.

Anyway...the idea was not that minus Anthony the two of them would be 'fishing together'. Far as Fredo was concerned, Al was there to bring out the boat. May have happened that way dozens of previous fishing trips.

Look, what's with you people? Even the stoic Tessio suffered a mili-second of panic when he realized the end was near. There is no way on earth the cowardly, naieve Fredo would have just realized he was about to be murdered and calmly said...let's go Al, then even more calmly thrown out a line and recited the Hail Mary. It's just not what Fredo was made of.

He just didn't know.

Apple