I agree that the Hail Mary was purely artistic. And I thought it had a good effect.

But think about it - we all remember how Fredo reacted when Vito was shot. He completely lost his ability to function, and he wasn't even the victim. If he actually suspected danger to himself, he would never have gotten into that boat. I sort of see Fredo as a child. I doubt if he would have taken the knowledge of a plot on his life with calmness and composure. No, he couldn't have known.