If Fredo knew what was about to happen why would he stay in the boat?
3 reasons for staying in the boat-
1) he was ready to die for his crimes; in which we see Fredo acting bold for the first time on film; or: 2) he was not ready to die, but knew he had little choice in the matter and decided to die like a true sicilian; or: 3) he did not know and simply wanted to fish