Infrequentflier 511
Do you mean Lidar? Green laser Lidar can penetrate up to 70 metres or so depending on the properties of the water (and atmosphere) so yes it can be used to detect objects under the surface. My understanding is that a swathe is recorded which is dependent on height of the aircraft above the sea. There will be an optimum height for maximum penetration but clearly the lower you fly the narrower the swathe. Its an angular thing.
Another complication is that other sensors on the aircraft will benefit from height eg if you are looking for a large object then you might want to fly higher and this would conflict with a deep penetrating Lidar survey.
They will also be using other sensors such as radar and various types of photography.