I have no idea what sonar frequencies are what, but to expand on your point, sonar is basically the only kind of sensor that works underwater. Radar and sight are basically useless.

Sonar is really all you have to see underwater, and is used heavily for scientific purposes, not only military purposes. Ocean-floor mapping is done with sonar. Active sonar of the kind that hurts whales is not in frequent use by deployed military vessels on military or surveillance missions because, while it gets you a good picture, it also announces "HERE I AM!!!" to the entire world.