Sound doesn't travel as well through the air, but it still does. The primary reason that you can't do it from the air is that the surface of the ocean is a big

Sound doesn't travel as well through the air, but it still does. The primary reason that you can't do it from the air is that the surface of the ocean is a big irregular reflector of acoustic energy from the air, so the overwhelming majority of the energy will be lost. It just would never be practical. It might be possible to use airborne gravity gradiometry to map seaborne mounts. I'd never considered that before, but I see no reason to think it wouldn't work.