[atoav]

This is correct, the different temperatured layers of water typically develope quite well defined borders, these border regions act like any border between mediums with differing densities: they refract and reflect waves passing through them.

[XorNot]

Right: but even in then, you'd still be able to drop a microphone down through the layers and rapidly survey the whole thing at once. Much easier if the target is making a racket.

[atoav]

Good point, that could be used to the searchers advantage.