So, the technique does not appear to work at all without the fishbone necklace equipped, even when near water like the docks. Possibly it does something while underwater, but I can't immediately check that. Maybe something like Ocean Song?
It looks like it's probably making a distinction between opponents that breathe air versus not. I got the choking gasp result against human, animal, and ghoul opponents, as well as a twisted hound. I got the increasing pressure result against a lake serpent, flying eye, writhing mass, and trash golem, so far.