Low Fliers excellent post deserves extending to make it explicit.
We need to sonar side-scan(SSS) an area of about one million square km to be reasonably confident of finding the remains of MH370.
The approximate ship speed and width of scan are known, so a rough estimate of the scale of effort needed is simple. The SSS beam width is one km at most and the ship moves at 10 km/hr, consequently the effort needed is about 100,000 hours of ship time. That translates to about 10 ship years, so a determined effort with 5 ships should be fruitful within two years.
As a side benefit, the world gets a superb high resolution map of a large chunk of the Indian Ocean sea bed. A wonderful oceanography project which could easily be combined with more extensive geophysical measurements. Oil companies might even be willing to help underwrite part of the search, as the data might help guide the search for hydrocarbons in the area.