To me Neri shooting fredo seems like a bad choice at some point his body is discovered, either through the lake being dragged either for him or somebody(thing) else or it washing up on the shore. If their explanation was that he drowned, neri should have whacked him ( no pun intended) on the back of the head with an oar, or overpowered him which would have been very easy and smothered him. It would have been very easy to explain this away as a drowning than a bullet to the back of the head. Plus the fact of the gunshot being heard. The only reason for that that I see is mike knowing it was done. Why would he doubt Neri would not do it. There would be too many people in the area of the lake to hear to gunshot for it not to be suspicious.