At 16m you can dive with Nitrox40 (oxygen enriched breathing air with less nitrogen) and the ground times will be mainly limited by fatigue and body heat loss. So it will be more the water surface conditions and temperatures which will determine the possible dive times. Since the small CVR unit can be deep in the mud, it can take a long time until they find it. They might at some time change to the typical gold miners solution with water and dirt pumps and separation on the surface.