Real Life answer
In real life the layer - if strong enough will form a surface half channel and keep the skimmers signature bundled between the layer and surface. This is also called a surface duct. It would have the same effect on his active too. Some of the sound rays will escape depending on the angle they interact with the layer and will be detectable. There is more to it but that is the gist. Game mechanics wise who knows (developers probably). I generally do not depend to greatly on what the sonarman says as he is operating on a very limited system.
__________________
|