Underwater horn

On the new brass hose nozzle I have, certain settings cause an alarm-like whistle. One can adjust the pitch by kinking the hose and altering flow rate.

I propose that a horn functioning purely with flowing water would have many uses. A simple one would be to signal to people doing things underwater over an area - for example many divers involved in some recovery operation: codes could blown (blown?) from beneath the boat to signal all of them at once. I suppsoe this could be done with speakers just as well, but probably not as loudly. Also, speakers require electricity but the horn would work with a pump and so could be mounted in a raft, immune to immersion.

Another use would be for long range communication, although really this is a seperate idea. If it works for whales it should work for robots. Low frequency sounds (morse code? fax talk?) would be broadcast, picked up by relay stations and rebroadcast, and so on. Each relay station would be autonomous, small and possibly submerged. Such a system could still function to allow long distance communication in the case of war or disaster: cut wires / loss of satellites / EMP pulse. I think these things could be rigged to work purely on mechanical energy.