It has to be sound. I think higher frequencies = lower range but more bandwidth and vice versa. Idk about ultrasonic. A trigger requires barely any data. So I'd guess lower frequency sound waves.
Any kind of underwater communication is strictly regulated. Someone told a story in a HN comment a few days ago about someone seeking a patent for something to do with underwater coms. The patent was denied, the DoD got involved saying this tech is classified, stop using it immediately. Any ropeless fishing the gov would be involved.
And besides, the anchor is on a chain anyways, why not the traps? What's the difference? We're not using wireless anchors