As you noted, DMC, this topic has been done many times. Some people can make a case that Fredo should have known that he'd be killed, or even that he might have known that it was his last ride, and went along with it out of resignation and/or repentence. But I think that, while Fredo should have recognized that his brother never, ever gave an enemy a pass, he probably didn't think of himself as an enemy, especially after that embrace at Mama's wake. So I don't think he knew.
As for Neri: Michael obviously designated Neri as Fredo's killer after the famous boathouse scene. I'm guessing that Neri (with or without Michael's advice) figured that, sooner or later, he'd not only have to kill Fredo, but find a way to dispose of his body--and create an alibi. I'm guessing that he figured that the lake would be a good place to dispose of the body. So he appointed himself custodian of boats--the chief boat-operator--to establish that he'd always be in a boat when the day of Fredo's death came around. Of course Fredo could have operated a little motorboat by himself, but he probably got used to having Al do it.
Others have pointed out that dumping Fredo's body in the lake was problematic: wouldn't the authorities drag the lake looking for him, and find him shot to death? I don't know the answer. Perhaps, with no witnesses, Al may have claimed that he brought Fredo back to the dock, and that Fredo then took off for parts unknown. We have to give FFC some license here.