Can we please stop talking about submarines being used in the search? A conventional, military size submarine will NOT be operated under the surface in 50-100m of water. It would be akin to trying to stay VFR in a widebody with a cloud ceiling of 2500ft. A typical submarine is at least 20-25m in height from keel to top of conning tower or periscope. Even running at periscope depth would be far too risky due to chance of collision with surface vessels.
Submarine sonar systems are specifically developed to be used when running submerged, surface turbulence renders them useless, so they will only be used as another vessel running on the surface searching if they are even deployed.
Specialist vessels such as hydrographic or exploration vessels with accoustic/sonar/MAD technology will be far more effective.
Having said that, as these systems search on a cone (and will be lucky to be travelling at 6-12 knots), and that the water is so shallow, they will be greatly reduced in their effectiveness to cover a reasonable area.
Given that the water is so shallow, the most effective thing if no visible debris is found would be specialist aircraft such as the Orion if fitted with the MAD booms. They will also cover much more area much more rapidly than any surface search using this technology.