[dogma1138]

AF447 Is a bad example they found bodies (first 2 were found within 5 days of the crash) and some wreckage pretty much immediately. In this case the last known position wasn't "that off" our understanding of currents and ocean topography was.

Even if Radar will give you the exact point of impact with water (which it will never do) there are still so many variables that it might not be that much of a help to begin with over a general grid area.