I remember researching this in 2008 and I thought satellite based lasers were already in common usage (sending signal to specific known surface location and also receiving from that location using the surface of the ocean as the medium that the sub and satellite read.
This was a while ago so no links maybe I’m misremembering.
What the other comment said. I guess it’s also because of friggin lasers and I may even be misremembering that part. But a laser would have a smaller beam diameter and less likely of interception.
This was a while ago so no links maybe I’m misremembering.