Why would the Law bother "dragging the lake bottom looking for [Fredo's] body" when the FBI and the Senate committee never did, Fredo alive! in spite of Fredo listed as Underboss in their Michael Corleone Family chart, to -

1. float like a butterfly -- Michael's welcome party [Moe Greene meeting]
2. sting like a bee -- conspiracy with Roth / Ola
3. sing like a canary -- ouch! Fredo already had

How is Fredo Underboss? The kid's clean. He was banging cocktail waitresses two at a time! He's never been busted for the rackets....